The absolutely most important thing to know about working on a garage door is to use ONLY 100% acrylic paint. NEVER use any oil based or alykyd based paint or sealer.
So the first thing is to prime the garage door. Typically new doors like this one have a white paint on them. It is an aluminium or steel door that has been painted with a factory finish on it. So what I do is prime it with a primer called Benjamin Moore Fresh Start, that is an acrylic based primer for interior slash exterior and you can paint.
What I like to do is tint the primer before I paint with it. If you can't find a paint store that will do that you have to paint on the white primer and then put a basecoat on before wood graining.
A really good basecoat to use for a walnut look is a Benjamin Moore Peanut Butter or one called Dark Mustard. It will look pretty gold when you paint it on but don't worry about that. The reason you want a very gold base is it will give the wood look that warm undertone it needs to look authenic.
If I am trying to make the door look more mahogany or cherry I use a Sherwin-Williams color called truepenny 6355. That will make the door look like a pumkin at first but it gives the final wood tone you want. Typically when I use Truepenny for the cherry look I use a SW color Fiery Brown for the actually wood tone.
You want to use acrylic paint and not oil. You can use a similar color paint from Benjamin Moore using their Mooreguard line for exterior.
Back to the walnut colored door. I use a SW color called Java for the wood tone. (By the way use exterior paints.) A quart is way more than you will need to do a double garage door. After painting on the grain pattern with the Java color and it dries I take a black and mix it with the Java about one to one. Then I go over the same area again with that thined down a little with Floetrol or use some of the clear seal product that I use to thin the paint. That product is called Varathane Spar Urethane Waterbased. I buy it a Lowe's.
At this point you just want it to be a transparent pass like a glazing effect. I have found out that using this two color pass technique really makes it work to where the door will look real.
Take a look at the first picture.
In the next picture you can see what I mean better.
A caution here is only use the blue tape that says its for sensitive areas. Don't use the regular blue tape. Because typically like this job I only have about 30 minutes to an hour between coats so the regular blue tape would rip off the freshly painted paint. In the last picture you can see how I paint in the horizontal grain pattern.
After it is all done I top coat it with two coats of the Waterbased Varathane spar urethane.