I'm fairly certain the chemistry set is progression locked (on the main quest). I don't know if it's progression locked by individual characters. I used a save which had almost everything done.
Once you've found the chemistry(/alchemy) set, you go to the garage (in the afternoon I think) and then click on the table. You then read the instructions, and then add the steps in the book. I can't remember how many you get that you can't repeat (I think it's one for Rae, Cait, and Lyriel), but you get three you can repeat. Those three will be available as choices after you click the table.
In terms of gameplay, they are just a sequence of events that happen afterwards. Repeatable for the repeatables, obviously. My guess is that it's going to be a pathway for repeatable actions, which is a decent mechanic. I'm not sure there's anything particularly helpful/meaningful in terms of lore; if there was, I didn't notice it. (Though the game leans into a lot of books, television, and movies, so knowing those may mask something that was hinted at or even outright said.)
I'm going to do another playthrough and I'll put all of the potions in a post when I'm done (I posted the repeatable ones earlier). The text isn't that bad (they're not hard to do), it's more that a single mistake wipes out the instructions in the book, and it can get annoying if you overthink it. If you want to give them a shot, just make sure you read each paragraph before adding steps. I'm fairly certain that instructions don't carry into the next paragraph, but I'm working off memory; I just remember that there were a few instructions which could either be 'applies to all ingredients' or 'applies to the next one'.
