Your canteloupe is ready to harvest when the joining stem turns from green to brown. When it's brown, you can harvest it. Don't cut the stem flush, leave a couple inches to protect the inside from invading bacteria.

Watch this video for an actual demonstration. I know she talks about butternut squash, but the same applies to canteloupe.

https://www.youtube.com/watch?v=VeYQ9GLMW4E

welcome to GoY.
I have always 'knocked' on the bottom and if it sounds particularly hollow it is usually ripe. some also gently push the bottom of the melon [other end to the stalk] and if it gives slightly it is ripe.