Jack up each rear wheel in turn. On the back of the brake backplate at the rear is a slot covered by a small tin cover. Move this aside and you will have a star wheel that you move round with a screw driver. Adjust until the wheel is locked solid. (I then stamp on that brake and see if the wheel turns, if it does, tighten the adjuster until it locks again). Then back off the adjuster until wheel just turns. Repeat on other side.
You can also remove the tin cover on the brake drum, there will be two large holes in the drum which gives better access to the adjuster.
When you have done this, lock the brake pedals together, select 1st gear and drive forwards at the same time applying both brakes. The tractor should continue straight ahead but if it pulls to one side, tighten the opposite side brake a little.
(Remember to block the wheels to stop the tractor moving when you jack it up).