Mavis,
First clean all the old belt residue from the motor pulley and the hand wheel belt grove. This helps give the belt good traction and prevent slippage.
Then loosen the screw in the center of the two large nylon bushings with the motor bracket running between them. To do this slip a screw driver through the hole in the right end of the machine's body. You can see both of these things in the second picture above.
Procure a new belt, (mine is an Alpha Sew #1514, you can get it at Sew-Classic or your LSMG) and put it on the hand wheel grove, then push the motor as far up as it will go and put the belt on the motor pulley. It's the big brown thingy with the "V" shaped groves in the first picture. Work the hand wheel back on and put the stop motion washer and knob back on.
Then adjust the belt so it's just tight enough to not slip, but no tighter.
That's basically all there is to it.
Joe