I've been asked many times for this, so here are some simple instructions to follow for mounting donuts:
Visit www.mcmaster.com and order part number 75425A67 (rubber contact cement). Paint a coat on the inside of the donut & a coat on the outside of the wheel. Be sure and rough up the outside of your wheels with 40-grit sandpaper. Let both dry overnight on a sheet of plastic. If you don't want to order the glue then any good rubber based contact adhesive at Lowes or Home Depot will work. Purchase a quart of MEK solvent at Home Depot or Lowes. Also buy a pair of thin chemical resistant gloves. In a well ventilated area, pour enough solvent into an old pot to give you 3" depth. Hold the donut in 1 hand and the wheel in the other. Dunk both at the same time for 2 seconds in the solvent. This will slime up the glue and you can slide the wheel inside the donut. Center the wheel up so you have equal amounts of foam hanging over the wheel. Let this dry overnight on the sheet of plastic. Then the next day you can chuck up your tires in your tire truer and use an xacto knife to trim off the excess donut hanging over the wheel and true them down to what ever racing size you are using.
WARNING: MEK is nasty stuff. USE IN A WELL VENTILATED AREA OUTSIDE. Use the chemical gloves every time. You can pour the MEK back into the can when you are finished.
WARNING: When using the Xacto knife be sure the wheel is turning toward you so the blade will go down if you let it slip.
WARNING: Don't leave the lid off the glue very long it will evaporate. If the glue thickens up over time just use some MEK as thinner.
Hope this helps.