The long time standard for many climbers to make a loop from tubular webbing has been the water knot. (Technically speaking it’s a bend rather than a knot, as it connects two webbings ends.) Basically a rewoven overhand, it’s fast to tie, easy to check, and reasonably solid.
Below - water knot in webbing
However, some recent testing has shown when this knot is loaded and unloaded, the ends of the webbing can creep, and eventually creep through the knot leading to potential failure. Also, this knot can come untied rather easily if one part of it gets snagged in a rock nubbin, tree branch, of other similar protrusion. See a video on this here.
What's a better option? The trusty double fisherman’s, typically used to tie a loop in cord, also works great in webbing. Don't plan on getting this untied, especially if it's been loaded and unloaded, and gotten wet and then dried off. It may take a bit longer to tie, but the resulting increase in security is probably worth it.
Below - double fisherman’s knot in webbing
So, if you still decide to use the water not, leave long tails of at least 5 inches (yes, longer than what shown in the photo above), tension the knot as hard as you can, and check it often, especially if it's used as a rappel anchor.