Hi Roger, this can be a whole topic by itself with lots of opinions, and there were a few threads in the past that debated how much to practice or not for Fundies, so you can also search if you'd like. The concern is that if you practice in an incorrect way, it will be harder to unlearn bad habits rather than starting fresh. Sometimes brand new divers do better in Fundies than ones who have picked up bad habits. Practice is great, but "imperfect practice makes imperfect".
Something that my GUE instructor told us to do prior to the course was to practice holding static trim at different depths for long periods of time, whether in a pool or in open water. So for example, staying perfectly still and horizontal at 10 (or 15) feet for as long as you can. If you can do it with a friend or someone can take pictures or video so you can see what you look like, that would allow great feedback. Then hover flat at 5 feet for as long as you can, then 3 feet, etc.
Once you have that down over multiple sessions, you can put them together. 15 feet perfectly flat and still for 30 - 60 seconds, 10 feet, 5 feet, etc., achieving neutral buoyancy at each depth before you move on the next, slow and controlled, exhaling slightly of course. We begin our ascent by inhaling, not by kicking, so you can practice that too. Your feet should be perfectly still, not sculling. If you have all of that down, when you eventually add task loading in your course, you'll already be really solid.
You could practice clearing your mask while maintaining trim and a particular depth, and then the same while doing a mask remove and replace.
Sometimes my buddy wasn't able to come to our practice sessions and I would get frustrated and didn't want to practice myself, and told my instructor, and he said no worries, face a tile on the wall in the corner of the pool and do it yourself - and I did.
Something else you can do is to check that you're balanced. Get neutrally buoyant at say, 1 foot off the bottom and stretch out horizontally. Don't try to make any corrections. Just lay out and see if you stay like that or tip head down or feet down. If you stay perfectly horizontal, awesome. If you're head heavy or foot heavy, this tells you whether to try moving your ballast down or up your body. That ballast could be where your lead is, the position of your tank, the position of your backplate, composition of your fins, etc. to bring you into a naturally flat position. You will learn that your body pivots depending on how far your arms are in front of you, the position of your head, and the position of your legs and fins, etc. and how to use your body to maintain trim. It's a lot easier though, if you aren't fighting what you can adjust with your ballast positions.
You can also check that you can reach and manipulate your tank valve when you're standing up, and also when you're horizontal in the water. If you have mobility issues, there are exercises you can do to help reach your valve(s), that will assist prior to doing the course when you have to show that you can.
What may not be a good idea to practice are actual procedures. Doing actual S-drills, valve drills, deploying an SMB, precise propulsion techniques, etc., are usually not recommended until an instructor has demonstrated the current procedures to you. This is where the imperfect practice can interfere with your learning. Even the GUE videos may not be the newest procedures that you will be taught in your course. Unfortunately, the procedures do change, even if slightly.
When you register for your course, you will have the opportunity to download all the course reading materials, which will include all the Standards and the Standard Operating Procedures. Read the SOP well, if you like to read that kind of stuff, because it will give you all the current steps in each of the skills that you will be learning.
Those are many of the things that you can practice, and perhaps not practice, IMHO. Good luck!
There are lots of mentors that could help you. If you're on Facebook, this is the GUE Quebec page which you could join:
https://www.facebook.com/groups/1587824734765359/
Or the GUE Ontario page which you could also join:
GUE Ontario