It isn't that hard Planemad. It's only a step or two above what you've done so far. When in doubt practice on some scrap material. Don't worry about cutting the trailing edge taper with a knife, use the sanding block and sand it in. Doesn't take much longer. Or do half and half to get it close and sand it to right which will save time. I seem to remember the dimensions were a little weird on the plans but were in the video but get some scratch paper and lay it out full size. At 12 I was taking drafting in Jr. High (back when we called it Jr. High) so age wise you're right there. Once you get over this step you'll realize taint nothing to it AND you'll have another accomplishment. Screw it up? Big deal. A couple more sheets of Dollar tree foam, heck, get some old big cardboard boxes from the Grocery store and use it to get dimensions close. For that matter, you probably could swap the two if there aren't any seems in the wrong place. Most items come boxed in big groups and the store just crushes the boxes up and ships em to a recycler. I know money is tight at 12. I hate seeing you spend 20 bucks more then the cost of materials on something you can do yourself.
While you CAN build successfully with a snappable blade knife, a Xacto knife and a 10 box of replacement blades PLUS a small whetstone was really the best, first tool I bought. I highly recommend one. I think they're in the same isle at Walmart with the mini hot glue guns which are usually only 3-4 bucks tops.
