Like many people who play the pipe organ, I'm not a great instrumentalist. And like them, I've not undergone formal training. I simply understand enough to recognise the real priorities for any church organist:-
(1) To play the music at the right volume. Not so loud that it deafens the congregation, and not so quiet that they can't pick up the tune.
(2) To use the stops to add some texture and character to the song.
(3) To support worship, and not to try to show off.
Before you begin, get an experienced organist to show you how to use the organ. These are expensive and cumbersome instruments. They were hand built, and so should be handled with care. Some aspects of your organ will be unique: how to switch on (and off), how firmly (or gently) to pull the stops, etc..
Keyboards
A normal English church organ has two or three keyboards. Any others are not really suitable for use in church services - unless you're an expert.
The top one is the swell organ. It has the great advantage that you can control the volume with the foot pedal. (A bit like an electronic organ.) So this means that you can select virtually any of the stops, and the volume won't overwhelm the congregation. And you can lots of stops together, and still play quite quietly.
The middle keyboard is the great organ. This is the loudest part of the organ. For normal church services, you only use one or two stops at a time. On great occasions, you might select a couple more - but ideally not, unless you're a skilled organist and you know what you're doing. (In which case, you shouldn't need this article!)
The bottom keyboard, often omitted, is the choir organ. This generally has a quiet selection of stops, controlled (like the swell) by a foot pedal. But beware of the odd stop which might not be controlled by the pedal. On ours, it is a VERY loud Tuba(!)
Then, there's the pedal organ - the keyboard which is operated by foot. For classical music, this is really important. For hymns and songs at church services, it's a lot less important. If you're dodgy when playing it (like me), you can skip playing it.
Stops
The stops on your organ probably have exotic or foreign sounding names. And these are supported by odd-looking measurements. Don't be daunted. But do try out each stop individually in an empty church before you use it in an actual service. Some stops could be unexpectedly loud, or unexpectedly quiet - or not operate effectively through the whole range of the keyboard. And some of the sounds might be very unpleasant if not used properly - or if the organ is not well maintained.
The key to stops is to classify them, into one of these classifications:-
(1) Reeds. The loudest stops. They often sound rough in their tone. Except on the swell organ, these are too loud for the novice to use. Sometimes they are too loud for anyone to use.
(2) Principals. These are loud. They sound like a classic organ sound.
(3) Flutes. These are quiet, and ideal for the novice.
(4) Strings. These are too quiet to support singing, normally. But they are good for playing at the start and end of services.
But how do you know which stop is in which category?
Reeds include: basson, bombarde, clairion, clarinet, cornopean, dulzian, hautbois, krummhorn, oboe, trumpet, tuba, vox humana. Some of these are MUCH louder than others.
Principals include: diapason, fifteenth, geigen, octave, twelfth, quint. Virtually anything which has a roman numeral in place of the normal number (e.g. IV, III) is a principal, as is virtually anything which has a fraction in place of its number. (Oddly, the stopped diapason is a flute, not a principal.)
Flutes include: bourdon, clarabella, gedeckt, nachthorn, nazard, piccolo, stopped diapason, and anything ending in - flote.
Strings include: cello, duciana, gamba, viola. And the voix celeste, which should be used with one of the other strings (and no other stops).
What about the measurements?
On the keyboards, you select first an 8' stop. Then, you might add to it other stops, to adjust the sound. Bear in mind that each added stop will increase the volume.
Adding a 16' stop will add depth and weight to the sound. It makes for a more serious tone.
Adding a 4' stop will make the sound lighter.
A 2' stop will make the sound more "churchy" - more like the classic organ sound. The same will occur if you select anything with a roman numeral or a fraction. But note that these stops are all LOUD. They will overwhelm your playing if the 8' stops are not strong enough. So you need to use them with care. And on some organs, they have a rather discordant piercing sound. So if in doubt, avoid them.
On the bass pedals, if you are competant, select a 16' flute as your basic instrument. Add one more stop, an 8' flute, if you want more volume from them. Do not dream of selecting a 32' foot stop. It may sound amazing to you, but it will shake the entire building, and drive the congregation out of the church! I once played a musical joke on a few friends, by selecting a quiet 32' stop during a song which mentioned the earthquake when Jesus died. I could not finish the song because of the laughter. Ah, well.
But do not use any reeds that may be available on the bass pedals. They are just too loud. And if, as on our church organ, you happen to find a 32' reed, never go near it. Not even in practice. In our church, the windows rattle. In others, objects could fall and break. And the neighbours will complain! (This is not a joke!)
But, even if you are capable, think about whether you want to use the bass pedals all the time. Omitting them for a verse of a hymn adds character. And they really don't go well with modern songs, where the bass lines are usually very weak.
Selecting stops in worship
I like to set up my stops at the start of a service, and not to vary them too much. I wouldn't very at all during a modern song, unless I was unhappy about my original choice, but I would vary for each verse of a hymn, to add character.
For example:
Swell: 8' principal, 8' flute, and a 4' flute. This doesn't sound too churchy, and will support singing without being too loud. If the sound is suitable, you could keep to this for the entire service. You can add a 8' reed when you want a verse to sound more powerful. Or, you can remove the 8' principal for a quiet verse. Or, you can add a 2' stop to add a more churchy sound. And on the final verse, you might use the reed and the 2' as well as the other stops. Try to avoid using the foot pedal to vary volume too often. People like a constant sound.
Great: 8' open diapason no. 2. (Note: Open Diapason no 1 is often too loud for congregational worship.) Or you might prefer the Stopped Diapason (but it is too indistinct on our organ to use alone). You can add stops just like on the swell - but bear in mind that the volume is much greater.
Another approach is to select the reeds on the Swell, and an 8' and 4' diapason on the Great. This is a louder arrangement - but you can switch between keyboards for different verses. And if it's not too loud, you can add a 2' or another 8' for the final verse.
Another approach is to use different keyboards for melody line and harmony. The idea here is that you select perhaps a trumpet (or another stop with a clear, distinct sound) on the Great, but only play the melody line. A selection of stops on the swell can support it, with the left line playing the chord. This sounds rather good with modern songs.
Song structure
Read through the hymns first, and decide whether you want to emphasise any verses by playing softly on flutes, or by adding a swell reed, or adding a 2', or omitting the bass pedals.
Remember that usually the last verse is triumphant, so should be loudest. But this isn't always true.
Get a reputation for playing gently. You can make metaphorical fireworks fly on your organ - but don't. This is a church organ, not a theatre organ. Theatre organs have (literally) bells and whistles. But a church organ is for worship, and worship includes quiet prayer. Many hymns are very personal in their nature. You should respect this.
And let the congregation worship. Don't overwhelm them with your playing. Keep it simple, make sure that you can hear them.
Develop a style of playing which lets people know when to begin singing. I play an introduction, then pause briefly, then begin. And at the end, all organists elongate the last note; because it sounds right.