Woodworking requires wood, a few assorted tools, an idea or plan, and some gumption. If you have all that, you CAN work wood. Fortunately for us we don't have to begin our woodworking endeavers at the tree, we can run down to our home improvement store and buy lumber ready to work, sort of. 1 by 6's aren't really 1 inch by 6 inches. 2 by 4's aren't 2 inches by 4 inches. Square cut board ends aren't square and not every board you see for sale is worth buying. OK, now that we have the confusing part out of the way we can get started. What? You want to know more? Well, lets look at things this way, every board you look at is a lie. It may not have started out that way but that is how it is. You can buy ANY board you want provided you understand that truth. The board is not what it seems and won't ever be unless YOU make it so. This doesn't mean you can't buy lumber to build whatever you wish to build, it simply means that it is up to you to make the board usable. If you want it square, YOU have to make it so. If you want it 4 inches wide, YOU have to make it so. If you want it 3/4 inch thick, yup, it is up to YOU to make it so. Having a chunk of wood is only the very beginning, it is up to YOU to turn that chunk of wood into something.
CRITICAL TIP: BEFORE you start taking measurements from the board ends, ">use a square as a guide and trim the board square. Mark the side of the board you took square from and remain constant from that point on for the entire board. YOU are saying "this edge of the board is good for squaring from, the other edge is suspect." This won't make things perfect but it will help. In reality, YOU should ">plane or joint one edge true to one face so you KNOW you have two surfaces that are good. As you improve on your woodworking and have tools to work with, you WILL do that. For now, this is a basic, get you going with woodworking article.
OK, you have trimmed the board end and it is now square and ready for you to take measurements from it. Take your measurement, line it out square and ">cut it. Pay attention to where your sawblade cuts and make sure the cut edge is where you want it. The width of a sawblade can really mess up a plan.
Taking measurements isn't difficult but does require a certain amount of precision. Folding rules (old school methods), ">retractable tapes, or ">fixed steel/aluminum rules are just a few of your options. WHEN I can use a fixed rule, this is my prefered method for measuring, otherwise I use a tape. I will resort to a folding rule when I have no other option since I just don't like them. What you use is YOUR call, just be as accurate as you can be.
When making saw cuts, USE a guide and your cuts will be better. YES, you can freehand your cuts and get pretty good results, with a guide used correctly, you get great results. The gist of all this is whatever YOU do with your wood, do it with accuracy and you will ultimately have less problems. Develope the habit, SQUARE is next to Godliness. Cut it SQUARE, BUILD it SQUARE. Go practice some square cuts!