I had some spare bikes so i thought i would make some ski bikes and ths is how i did it.
I took the hubs off the wheels and split them I cut the hub in half with the spindle out and mounted it in a tube and put the spindle back in to hold it together.
Then i made this triangle shape to hold the blades (ski) and just put it back on the bike.

Step 1: Splitting the Wheels

Take the wheels off the bike and take out all the spokes so you are left with just the hubs.
Then take the spindles out and bearings and keep them in a jar or something these will need to go back in.
cut the hub in half so you can insert a steel tube between them then you can weld to the steel tube

Step 2: Making the Triangle

Cut three lengths of tube between 8 to 10 inches depending how high you want to be of the ground.
These must be welded together into a triangle with a small bit of tube at the top for the hub Togo in.
The blades or skis are bolted to the bottom part of the triangle then the split hub, spindle and bearings fitted to hold it all together.
Make 2 of these one to replace each wheel.

Step 3: Foot Pegs

After fitting triangle on the bike make some foot pegs.
I just bolted BMX stunt pegs onto the bottom bracket you could take the bottom bracket out and slip a piece of wood in.