If you need any help, I am happy to help however I can. Keep in mind, though, I have never kept a brackish tank, only FW and marine (reef) tanks. But, I think with my experience with marine tanks I may be able to at least point you in the right direction. If there is anything I can do, let me know!
On Fishlore, I know that Pinkfloydpuffer can probably be a ton of help to you. I haven't seen her post in a while, but maybe shoot her a PM? I believe she's kept GSP's and other brackish puffers.
First you need to correctly ID the puffer you are about to keep. There are some strictly freshwater puffers. Others need freshwater as juveniles. Then low-end salinity (brackish here we go!). Some stay there and do great. Others require an increase in salinity (still brackish). Some puffers are true marine species.
Pepetj - I am trying to decide between F8 and GSP.
This is my understanding on FW to BW
Freshwater to brackish is really easy, you just buy marine salt and a hydrometer or refractometer, add the marine salt to a bucket with water in it, use the hydrometer or refractometer to measure salinity to get it where you need it, let the salt water sit for a day, and then add it to the tank. you don't need any special equipment or anything, but just remember that salt won't evaporate from your tank, and make sure you test the salinity to keep things in check.