Actually, we were taught to patch test before each and every treatment - even for repeat customers, as allergies can develop in the intervening period. Just checked and it's in the training manual too, so if you don't do it you will void your insurance.
The plaster method is taught in the training. It's just as it sounds you put some of the solution on the plaster and apply the plaster! Be sure to use latex free plasters though as many people are allergic to these too.
If you are posting it out, you send the solutions and the plasters for the client to make up themselves -and Nouvea make an LVL patch test card with instructions. Hence why if you do this you have to send a whole set of solutions to a client (which you could of used to do 2-3 treatments within a 24 hour period). It's incredibly wasteful. Nouveau should sell smaller patch testing samples with just a tiny bit of the solution in at much cheaper cost... ie not enough to do a treatment with but enough to patch test.
I guess if you timed it right you could get the client to do the patch test with half the solution, seal up the packages, and then coming in for their appointment within 48 hours ( but not less than 24) and use the same solutions, but it doesn't seem very professional / hygienic to me saying - oh have you got those packs i'm now going to use them for the treatment!