Get yourself a spray bottle and make up a batch of solution according to the mixing ratio on the bottle. (It will smell "chemically")
Fill the "solution tank" of the spray cleaner with CLEAN water.
Hook up the vacuum, high-pressure hose, power cord.
Get some old rags - there should be an assortment in the back (green room). You might want to dampen 1 or 2 to start with (Just damp, wring it out).
Do not start up the fans yet (Other than the theater’s ceiling fans).
You will want to spread a larger cloth behind and under the chairs you have selected. These should be DRY (maybe paint drop cloths).
Spray 1 or 2 chairs with the spray bottle of cleaner, enough to be a bit damp but NOT soggy (a bit more than a light mist).
Using a clean cloth (possibly damp) wipe up any "overspray" that gets onto solid surfaces (metal that borders the chair, wooden armrest, wall behind the back row, etc….).
Let chemicals set for about 5 or 10 minutes.
Turn on the spray cleaner machine and use the clean water to extract. At no time during this process should the fabric have dried out. Will take several passes.
Depending on this process, you might want to apply again. If you think there is more soil to be removed. You will still want to wait for it to process. It is easier done now while the chair is still damp. And make sure you have extracted all of the chemical, continuous exposure is detrimental to the fabric.
Take an extra "dry stroke" - without depressing the water trigger to vacuum up all the excess moisture you are able.
Start small (maybe 4 chairs at a time). Do not forget to rotate the dry protective sheets - which will get a bit more wet as the time goes on - on to keep them working in the area where you are.)
Periodic Cleaner Cleaning
At some point you will need to refill the solution tank with clear water, this is a good time to empty the vacuum tank as well. Disconnect the hoses, take the whole thing outside and dump it on the gravel.
Drying Fans
Once you have done a few of them, then time to turn on the fan to blow across them. (Be careful, the fan will dry out ones that have been sprayed but not extracted. That is why you are waiting to do this.)
Completion Cleanup
When you have completed your session, make sure to dump out any soiled water (not good to keep water in the solution tank either) and rinse out any residue from the vacuum tank
Also, remember the cloths draped so the carpet and walls don't get chemical exposure? You definitely want to dry those out, and not in a wad. You might want to spread them out on the stage or whatnot, just to get a bit of air circulation around them.
IMPORTANT!
The sprayer is designed for non-sudsy products. Using some sort of foaming cleaner is a definite no-no. It goobers up the works. So, no dish soap. No laundry soap. No hand soap. Nothing like Fantastic or 409, or whatever is out there on the free market. NO FOAMING STUFF
References
https://sandiaplastics.com/3-gallon-spot-extractor-kit/ keep in mind the pump is 100 psi. Not the smaller one.