Hey man! I did a 7CS jetpack a few years ago and documented how I built it, the assembly process should be the same for yours and the paintjob is similar to the post sarlacc version. Here's my thread starting at the point where I began to build the kit, it should help
The time has come to begin the jetpack, the part of the costume that I've been most excited to do but at the same time also most intimidated by. Using a 7CS kit. I've begun test fitting, and cleaning up and prepping parts. Lots of pieces!
Seven Cat Studios also has a a video on how to prep, glue and assemble the thrusters in his kit which was super helpful!
There's more than one way to do it but the way I would recommend is to first prep all the parts with sanding cleaning up molding artifacts/edges/ect and paint them separately before attaching to the jetpack with epoxy or strong glue.
All the parts in green here I painted separately and once main colors of the body were painted I epoxied them on
There's indentation hole marks in the body where the rocket and side thrusters attach.
You can drill holes through those marks and then attach them with wingnuts and washers from the inside if you want to disassemble it for transportation.
SevenCat explains in his videos how to assemble the bolts into the thrusters, but for the rocket base I just drilled a hole and epoxied a bolt in there deep enough so it was secure.
Hope that helps! It's a fun kit to build!