Having a flat building surface is a must. Many people use a hollow core door for a flat surface. I put a ceiling tile on top of a door for building on. This give me a flat surface that I can easily stick pins into for holding things down. I also like to have some weights on hand to help hold things down flat on the board. What I have done and works great is to take a few zip lock plastic bags, say quart size and fill with sand or small rocks. These work great as weight and its virtually free.

The others have given you some good hints as well. Enjoy, building a kit is not hard at all.