After WTxRed mentioned tutorials, I got online to find some for myself. Most rely on square rulers, but I did find a very helpful one which I pinned. With the diagonal marking on my mat it looks as if I can make true squares with little problem. I might be cutting these blocks down some, which will probably mean a wide-ish border or extra blocks in order to fit the bed, but I do believe I can get them done so they are straight — which I hope are not Famous Last Words. Thanks to everyone. People
Thanks to everyone. People are fabulous about quick responses here and I really appreciate it.