Its really easy. The trucks are assembled with just two screws and you only need remove one to release a side plate. Then simply remove the wheels and axles and replace them with any 3-rail set using blunt end axles (most are that type).
The end frames are held with pins, so expect them to fall out, but just place them back in place and hold them in position when to screwing the end plate back onto the bolster.
The SMR 2-rail wheels are scale 32" diameter. Test fit your wheels to avoid anything larger or they will possibly rub on the truck frame. If you don't have any suitable wheel sets in your parts box, I suggest you order some from Northwest Short Line.
Dave