I’d say “get a professional” but I don’t think that would go down well. So instead I’ll say measure, measure, measure. Measure 5 times, 6 times before you cut, before you buy, before you make any permanent modifications. By far, the most mistakes occur when people don’t take the time to properly measure and make sure that they’re doing the right things, in the right order. Being off by even a quarter of an inch can be catastrophic.