Shaving with a straight is a slow learned process, regardless of where you're shaving.
When it comes to the head, go very slow and deliberate.
I would learn in sections, work just the top of your head one side. Once you have that, work the other side until you're able to get all the top without problems.
Now onto the back of the head (my least favorite place to put a straight).
Once again, one side until you get the technique down, then the other.
One thing a straight razor will teach you is that shaving with one is not a race. You rush and you'll bleed.
Small steps, it's a process...