Sorry, missed the question in the earlier post. The roots are cut the way that they are to supply the tree with enough nutrients and water. Topping the tree causes it to bush. This topping is not only important to cause the tree to tapper like a full size tree, but the tree no longer has a real top to continue upward growth so the tree grows more branches. You will need to deal with these as they come in.
Ok, so you decided to go with a tree from the area, a wise choice. Do not go for one that has too much height unless you are going to trunk it. Trunking is a short cut to a thick tree trunk look, but you have to hide the cut. One way is to let the tree bush around the cut or, my preferred method, design the cut into something else. You do this with a sharp knife meant for cutting bonsai or whittling. I usually make a large knot or make it look like half of the tree has broken away. You will need a dremel tool to clean up the look. I have an extra one I would be willing to trade, muhahaha. Over all I do not care for the look of trunking as much as not trunking.
Now for getting the tree. At this time, if you have not pulled it already, dig under the tree and cut the tap root. Leave all other roots intact and leave the tree where it is. Top the tree off. If you plan on a formal upright take one of the remaining branches, bend it up and tie it to the trunk of the tree. You will then have you new top. Let the tree remain where it is through the following growth season. At the end of the growth season you will cut around the side root and move the tree to a trench. The reason on the wait is that removing the tree and cutting all the roots is too much stress for the tree. If you buy a tree from a store (not a bonsai, but a real tree in a pot) you can go ahead and trim all the roots, top it and trench it right away. Make sure that the soil stays moist until the tree goes into dormancy or winter is upon you.
---------- Post added at 01:50 AM ---------- Previous post was at 01:43 AM ----------
Oh, if you do a formal upright on the tree check your tie often as the tree will grow around it and spoil the Bonsai.