Start at the very basics and work up. Thats what I do.

For example, the tank in my RTS was built step by step.
First I made it so that I could selected it. Then I made it so I could selected multiple tanks. When that was done, I made it so I could move it. When that was done, I made it so I could move multiple tanks. Then I made it so it can attack enemies. . . and so on and so forth. Start at the bottom and work up.

You need to think: what does the object have to be able to do primarily before it can do anything else? and get that finished first.