Keep your bonsai in the ground (transplanted to your yard of course) and let it get as tall as you'd like your bonsai. Then cut off the top and allow it to "back branch". This is the sending out of new branches from the older buds, preferably from those near the top for your tree. Then let those branches grow only as long as you'd like them... usually just short of the ground. Get yourself some Copper Bonsai wire. It can be ordered online. You'll need some thin wire for the thin branches of the willow. You'll need to check some sites on "wiring" to see how to do it correctly. You dont' want to cut into the branches soft bark. Wrap it around in a spiral fasion from the trunk to the tip. Bend the branches the way you'd like them to fall... As close to a natural looking Old tree. Check them every few months make sure the branches are not being cut by the wires. Remove and rewire if they are.
Every 3-6 months I would take a shovel/spade and cut a ring around the tree about the diameter of a bonsai pot you'd like to see your tree in, then slide the spade a few inches (about 4-6 inches) under the tree. This is to cut and trim the roots to the length needed for a pot in the future. Do not lift the tree! Just leave it there. It will recover much faster this way. After a few years of training like this, (and you can show people your tree in it's spot!) You can eventually move it to a bonsai pot of your liking. By this time, your tree should have a nice sized trunk and a good shape, if you kept up on your wiring and branch pruning.
This is not an overnight process. Creating great bonsais takes years.