One viewer asked how to bend the wood for garden arches. The reality is that most walkway arches are not bent wood, but wood glued together and cut out to the curve. But you can bend wood.
If you really want to bend thick pieces of wood, that is usually done by steaming it first. You could take a long piece of PVC pipe, close off one end and fill it with hot water to soak wood and get some bend out of it. Of course different wood bends differently, oak probably being one of the best for bending and retaining its strength.
The first photo shows one way to bend wood to a curve. The use of a strap to pull it around a curved template helps greatly to provide even pressure and prevent splinters. If the bend is radical, soak the wood first, bend it partially and let it sit like that overnight. When you release it from the jig it will retain part of its bend. You could soak it again and form it again. It is a good idea to build the template so that you over bend the curve -- that way it will relax back to just the curve you want. After you have conditioned the wood, or the multiple slices of wood to the shape, then apply the glue and bind it all to a jig that is in fact the exact shape you want for the final product.
For multiple curves you might want to cut the template and use both sides by pushing it back together as in the second photo. Don't forget wax paper under the jig so that the trim doesn't stick to the table, and wax the mould itself so that the trim doesn't become part of the jig with the final glue-up.
Some pieces don't lend themselves to using a strap. Bending around a corner with a long trim piece can be done with a clamping arrangement as in the third photo, which is in fact a jig for the front end of a canoe.