Here's how you can set up the pistons in advance in most cases:
1. Draw the desired manual and/or pedal stops.
2. Locate the "Set" button on the left hand side of the lowest manual.
3. Locate the number of the piston on the front of the lowest manual you will want to use.
4. Press the "Set" button and while holding it, press the desired piston, release it and only then release the "Set" button.
5. Locate the "Cancel" button on the far right hand side of the lowest manual and press it to cancel all the stops and prepare the next combination. You can skip this step if the next registration combination is very similar to the previous one. Simply add or remove a few stops by hand in this case.
6. Repeat the steps 1-5 for any other pistons that you want to set up.
Note that you can use the divisional pistons as well. The divisional pistons are the buttons designed to change registration only for a certain division or manual. For the pedals, there are toe studs. The divisional pistons can be set up in the same way as the general pistons.
Here are some things to keep in mind if you want to practice using pistons:
1. On the place of the score where you have to change the combination, write the number of the piston and circle it. This will differenciate it from the number of the stop. You can use sticky Post It notes or something like that in order to keep the score clean.
2. If the piston change is difficult, slow down the tempo and practice the same spot with the exact motion of the thumb over and over.
3. If your practice instrument doesn't have pistons, you can pretend that it does and practice the movements with your hands anyway. This way you can prepare for public performance on the organ with pistons quite effectively.
There are toe studs for pedal division and/or for general combination pistons on many organs. They can be set up, practiced and used in the same way as the general pistons.
Note that during the performance you can add or remove any number of stops by hand in addition to the piston system.
Some instruments also have a sequencer ("Next" or "Previous" buttons or toe studs) which can facilitate the piston changes for the organist. If you set up the entire combination system in advance from 1 to 10, instead of pushing these pistons by hand, you can simply press "Next" which would engage pistons 1, 2, 3 etc. Likewise, pressing "Previous" will result in engaging pistons in descending order.
As with any other hand or feet movement, using the pistons during the performance requires some repetitive practice to do it fluently. Don't take the piston changes for granted because in many difficult pieces, the changes can throw the inexperienced organist off balance, disrupt the flow of the music and provide the opportunity to make unwanted mistakes.
Simply regard the piston changes as an integral part of the performance process and practice keeping them in mind in advance. This way you can be sure that when the time comes, you will be ready to locate and press the required piston or toe stud successfully and effortlessly.