You can buy different sized pins. I have several different diameters/lengths of pins. When shopping for pins(Walmart, JoAnn's Fabrics, etc.) get the kind with larger plastic tipped heads. They are easier to push in and pull out. Then on the smaller/thinner pieces use smaller pins. Bigger pieces can take the larger pins. Get a sufficiently large flat surface(Lowe's and Home Depot sell inexpensive hollow core doors) and place ceiling tiles over that. Go to Office Depot, Staples or Kinkos and get a full sized copy of your plans. Place wax paper between your copied plan and the parts you are building. By doing this you always have the original plans to easily reference during the build and can save them after the build. If the parts you are talking about not fitting exactly are the square balsa sticks that criss-cross from the leading to trailing edge, as long as they fit flat against the leading or trailing edge, the location is not critical. If it is the pre-cut shaped parts, do as pmw stated, just fit them together. Shims should not be needed. If you are new at kit building, I would definitely use the plans to build on. I've built many kits but not with the plans on the wall. Maybe I'll get there, though