First find your belt. May have to buy through the internet these days unless Radio Shack still stock them.
On better turntables the platter is in two parts. You remove the mat and lift off the outer platter and thread the belt around the inner.
As I wouldn't tend to associate the word better with Realistic stuff, I'd assume this is a one-piece platter with an integral inner hub. Remove the mat and see if there's a gap in the platter where you can see the motor hub. The idea is to drape the belt over the main platter, perhaps stretching it slightly to fit, then thread it around the inner hub and the motor hub.
If not, the platter may be retained by a springclip or circlip on its shaft. Remove this and lift the platter enough that you can thread the belt aroud it and the motor hub. This may involve stretching the belt slightly to clear the platter. If there's no circlip the platter may be integral with the shaft.
Alternatively you may have to remove the botton of the turntable's plinth and release a screw in the bottom of the platter's shaft (the bearing housing) so that you can lift out the shaft.