Let that one be. That one is Not worth the trouble and will only be a pain to work with. If there's one this sized, you can bet there are more around, most will be alot better.
Look for an upright tree with a base that has some flare.--think base of a volcano-ish. Think 3-6 inches in diameter.American hornbeams close trunk chop wound V-e-r-y s-l-o-w-l-y if at all. It's a problem with the species and bonsai. Huge collected hornbeam almost always have some dead wood carving at the top that takes away from otherwise nice trees.
You dig hornbeam around here beginning in late Feb. just before buds break-the closer you can get to bud extension without actual leaves developing, the better. Mid-March can be prime time, but cold springs can slow things down. When you collect, simply measure out from the main trunk six inches, saw off major roots all around the tree with a hand saw or better yet a cordless reciprocating saw, Explore around the trunk again with a hand trowel or small shovel (shovels are useless in actually digging up a hornbeam.THey make a good prybar however.) Find the deeper thicker roots, use the saw again, push (the still intact) trunk to see if it moves. If it does, see if you can push it hard until the root mass is more exposed--apply the saw. Push and pull and move the trunk using the trunk as a level to do so. That can help break remaining old roots. Eventually with repeated sawing and leveraging, you will probably hear a "pop" and the trunk moves freely. That means you can lift it out and start sawing off root, including the ones you split by leveraging the trunk. Then top the trunk--typically on larger trees I chop the trunk to about 2 1/2 to 3 feet, depending on the diameter. Leave six inches more than you think you need--that provides maximum surface area for new buds to pop and use to create an apex. SEAL THE CHOP.
You don't really need ANY feeder roots with the tree, although two or three interior sets can help. You can usually saw off the main roots drastically, like to four of five inches. Make sure the edges of the cuts aren't ragged. You had better have prepared a reasonably-sized training pot (large bonsai pots can typically be used, since you can do drastic root reduction. Don't seal root cuts. Tie the tree in WELL. It shouldn't move at all. Fill in with regular good bonsai soil. BURY THE NEBARI BENEATH AT LEAST TWO INCHES OF SOIL--which will prevent it from drying out.
Water well, water well AGAIN. Set the pot in a spot where it will get two hours or so of morning sun--heat on the roots will help develop roots faster. Don't be tempted to mess with the tree. Don't make design plans or plot formal display plans for it, etc. You tree is only half-collected at this point. It requires time to recover. You won't have it out safely for another two years. Three years is minimum for beginning formal work on it.