For starters I want to point out that we have people on this site that have perfected many of the graphics, printing, and materials which makes purchasing a field cover a great choice.
For those who love the "do it myself approach", a very simple way to create a field design is using Excel (or any spreadsheet program). This kind of program has the "grid" that makes it easy to have perfectly drawn out lines. The technique for the lines is to set column and row width such that you have perfect square cells (each representing 1 yard). Then simply add "border lines" to the cells as necessary to create the field lines. Obviously the background color needs to be green and the cell border lines white. Then simply add graphics thru cutting/pasting and importing from whatever source you can find on the web. If you are familiar with Excel you will know what I mean and this is a piece of cake, taking less 15 minutes to create the general field outline and hash marks. If you are not familiar with Excel you would have to play around a little bit to get comfortable. The real challenge is printing and material selection. You need to find an ecomonical means to printing the large size cover. Obviously, it would be easiest to apply the field cover as one sheet, but its feasible to apply the field cover as multiple sheets of paper and simply spray clear coat over the top to create a nice smooth surface. If you don't want to do the homemade low cost paper (with spray clear coat) route you can go to sign shops or printing shops to talk to them about material options.
Joe