This is not a big deal but will show up in the final board.
First, you should cut the glass as evenly as possible so the lap line is consistent. This will make sanding it flush much easier. See how the glass it all wavy? That's more work to sand.
Second, you can simply use a hard block with 60 or 80 grit paper to sand down the edge of the fiberglass until it comes as close to the foam as possible without gouging the foam. We do it with a power sander and hard disc now but a hard block by hand is much easier to learn when you're starting out.
Also, another little trick instead of sanding the laps flush is to simply push the fiberglass edges down into the foam with a popsicle stick. Just enough to get them flush with the deck foam. Again, it helps to have a clean and even fiberglass cut to reduce the work and inconsistency.