How to Measure tank-top arm holes? Anyone who sews, knits, or crochets, can you help me?

I've made a few simple tops using ones I already have as patterns. If you have a tank-top with a similar cut, lay that out flat and draw a pattern from it(front and back) on a piece of paper or some scrap fabric and use that pattern for what you're making. Otherwise, in order not to screw up your cutting on the thing you're making, get some bargain bin fabric similar to what you're sewing with and experiment with it to get a pattern. Baste something like what you're making from that fabric, try it on, and cut away small bits at a time until you have the cut you want. Very useful to have a friend do the cutting while you wear the pattern piece.