For making a beautiful and natural looking aquarium there is nothing that compares to having some real wood as part of the aquascape. One of the problems with using wood in an aquarium is, wood floats in water. With most aquariums the wood will be wanted to be submerged, not floating at the top.
Wood does float but not always. Many types will actually sink if they are fully saturated with water. It can take time to saturate the wood but it is easy to do just by keeping the wood submerged for a few weeks. The time can be more or less depending on the size of the wood and the type of wood. Popular aquarium wood like manzanita can saturate and sink fairly quickly.
You can soak it in a bucket or other container of water, just be sure the wood is fully submerged in water. You may have to place something heavy on the wood to keep it all under water. Soaking the wood for a while is also good for washing the natural tannins out of it. When soaking it is good to change the water once every day or two.
If you don't want to wait that long and want the wood in the aquarium you can attach weight to it. A piece of slate or stone can work well. These can be glued to the wood with silicone, drilled and attached with a stainless steel bolt, and it can be tied with a piece of fishing line. Just make sure you use silicone that is non toxic or food grade silicone.
You could also just set a heavy rock on the wood but there could be a hazard if the rock and or wood slips. The falling rock and the wood rising to the surface could make a mess of your aquarium, break something, or even injure or kill a fish. Because of this the best way is to either saturate the wood or securely attach a weight.