I’d stop and think about several things before moving on.
Your rocker first, you need to know what your rocker will be. The rocker is so important to how well the board will move through the water.
Outline, the blank may already have the rocker you want if you can move the outline forward or backward, but you need to have the rocker figured out.
Bottom, flat, vee, concave, belly (convex). The bottom shape will make the board faster or slower.
Rails/profile, what kind of rails do you want? The type of rails can affect the profile or distribution of thickness. Having the thickness in the right places is also very important. The bottom shape will make a difference for how the board’s rails and thickness are setup.
I was told to plan out these variables and draw a lot of lines on the blank. Make guidelines every inch or so along the top, sides and bottom. Then go slowly and make your cuts to match the lines. Bill Barnfield says he sees the blanks as a wireframe. You can just draw the wireframe (sort of) on the blank.
I take a piece of foam and cut out an L shape. Then on one side I make a bunch of holes to hold a pencil at 1 inch, 2 inches, 3 inches, 4 inches. I do it at 1/2" intervals. Try to get the board close to the maximum thickness you want when you cut the rocker. Then use the guidelines for your cuts. Once you have the rocker set, re-draw the outline and after that is set, you can draw the guidelines along the rail for thickness. After the top is thinned approximately to the way you want it, draw lines on the top to cut your rail and profile. Make sure you leave a decent amount of flat area along the rail like an inch, or you’ll start cutting into the outline before you’re done.
I’d set the planer to very shallow cuts to start until you get the feel for it. You’ll need to get to 1/8" cuts, but start shallow. If you move forward slowly, the planer will make nice clean cuts. You just need to learn how to make the cuts even as you move from rail to center. You don’t have to hold the planer straight when you use it, you can hold it at an angle. Play with it and see how that helps with certain cuts. The top nose is the hardest part to cut with a planer because of the rocker. Don’t have any great answers for how not to ruin your blank, so be careful there.
If you mess up a little stop. Walk away and think about it, then come back. Don’t try to power through or you may make things worse. It’s easy to cut foam, a real problem to add.
Check out some of the youtube videos and see how those guys use the planer. Also, make sure you have a place to work where you can see the board properly. Light coming from the right angles allow you to see better. That’s really important to a clean shape.