DMoore has a great answer. Let me add that you want a bracket designed for this purpose. 3/8" with gussets. 1/4" without a gusset is way too wimpy, unless it is as wide as your overhang. Mine is custom 1/4", made up of several pieces that span the entire width.
I have gone through this on about 20 installs and have gotten better from my first ones. The thicker the L-bracket the better. A lot of time the big box doesn't sell ones that are made for countertop jobs. This is something I usually order online because I get these cheaper and better quality.
I am going to give you pointers based on you making these ...