I've got a few things that work really well
1: I print the plans and lay them out on the board. I pick a trimming pattern, and cut off the excess. For instance, I always remove the right and top trim from every piece, so I can line up the match marks by overlapping the next sheet.
2: I spray a light coat of spray glue of the back of each sheet and give it a few seconds to dry a bit before laying it on the foam board. It is easily removed and repositioned this way. I use Loctite 200 - the red can at WalMart.
3: These are the three MOST IMPORTANT tools: The exacto knife, the sharpening stone and the steel ruler. I have a cheapie WM knock off exacto knife that came with a bunch of blades. $5 on clearance after Christmas. I know most of you just change the blade frequently, and that works fine, but I have made a dozen planes with just two blades. The sharpener is quick and easy, and it works great!
4: Cutting: I always give the blade a quick few strokes on the sharpening stone before each cut. I use an 18 inch stainless steel ruler with a cork back to guide the straight cuts. Line up the ruler to make a consistent cut - Always be on the outside of the printed line, for instance. Sometimes you have to place the ruler inside the cut, sometimes outside the cut to support it.
5: Hold the knife straight. Seems obvious, but my first few sheets did not have nice square edges.
6: THREE passes - at least: First pass through the top paper with your freshly sharpened blade. Second pass through the foam. Third pass through the back paper. I have a thick piece of cardboard on my work table. Those rubber cutting mats are even nicer! Don't get greedy on your cuts, or you will get a ragged mess instead of a clean cut. Use more, lighter passes if it feels sticky. Sharpen ( or change ) your blade if it's feeling or looking wrong.
7: Finish up the free hand stuff - Curves and small stuff. Take your time and you will get the feel of it.
8: Don't worry about perfection! I just straight cut the small curved wing tips, for instance. That's fancy stuff for laser cutters... I did free hand cut all the scallops on the DR1 triplane though - that took a while, but looks great!
9: Peel off the plan from the foam board. Most always easy, but occasionally hard to find an edge to lift. I sometimes have to fuss a bit or pick at it with the edge of the knife to get it to separate. I sometimes lift a bit of the foam board paper. Just fix it and move on...
Good luck! You will build speed and skill quickly!