Hi!
Well ! Sanding is an art and you just have to learn it.
A good start is to have a long (30cm or longer) aluminum U-shaped bar and wrap the sand paper around it. Those T-shape aluminum bars from available from some hobby suppliers are no good.
It's also vital to have good sandpaper. The best sandpaper I have found during my 30 years in this hobby is a Portuguese sand paper from Indasa, called "Red line, Rhino dry".This sand paper is sooo good compared to everything else on the market, even compared to 3M sanding paper that you will be amazed.
It's available in many different grades from 60 up to 1200.

Regards!
Jan K
Sweden