Hi Dan,
For sanding the inside cuts - where I assume you are having the problems, you're on the right track. Maske yourself a little sanding stick out of quarter or half inch thick scrap (whatever size fits). I would make it wide enough to fit the ends of the rays but you can also make it wider. Glue a piece of sandpaper to it and use that. I usually put on enough sandpaper to cover about 1/2 the length of my 4"-5" stick. That way I have a fair bit of paper to use and plenty of wood to hold on to. And remember, the stick has two sides so you can use both.
After the initial straightening, you will probably want to repeat the process using finer grits. Oh, and if you don't have any scrap laying around, you can also use popsicle sticks or tongue depressors. You can get them for cheap at a craft store.
Pushing harder won't make it sand any better but it might make you break the piece. Firm pressure for sure but don't go all Incredible Hulk on it. If your lines are pretty wavy, you might want to use a more aggressive grit but be careful not to distort the final shape you're aiming for. You might call these lessons from unhappy experiences, lol.
As the others have said, straight lines take a bit of practice. If you stray from the line, move back to it gradually and it will be less noticible than a large correction. And this will sounds stupid perhaps, but don't forget to relax and breathe. I don't know how many times I caught myself holding my breath when I was starting out. It doesn't help! Good luck!
