That's just an excuse to learn how!
The
wiki page has decent information on it, if a little difficult to follow.
The example reaction should work just fine:
[REACTION:<identifier>]
[NAME:<name>]
[BUILDING:<BUILDING NAME>:<BUILDING KEY>]
[REAGENT:A:150:BAR:NONE:POTASH:NONE]
[PRODUCT:100:1:BAR:NONE:PEARLASH:NONE][PRODUCT_DIMENSION:150]
[FUEL]
[SKILL:<SKILL TOKEN>]
[AUTOMATIC]
[ADVENTURE_MODE_ENABLED]
Just give it a proper identifier (any string of numbers and letters, spaces, and underscores) and in-game name and a
skill token. You can remove the building, product dimension, automatic and fuel tokens. Then change the reagent and product strings to match what you want.
Replace "pearlash:none" in the product with GET_MATERIAL_FROM_REAGENT:A:NONE which will give the product the material from your reagent. Replace the item string (bar:none) in the product with TOOL:<your pouch ID>. The reagent token's item and material strings you can replace with BACKPACK:NONE:NONE:NONE. That will allow it to accept backpacks of any material. Replace the quantity (the value after 'A') with '1' so you only need one backpack, instead of 150.
In case you care, '150' in both the reagent and product dimensions tokens in the example are used by bars because a single "bars" item contains 150 bits you can think of as individual ingots or something. So in that case you're using 1 full bars item of one type to make one full bars item of another type.
This was REALLY hard to type up on my cell phone, and I can't troubleshoot it for errors on the phone so I'm sorry if I made a mistake.