How can I tell my watermelon is ripe and ready to be picked?

The underside will turn yellowy.

I always knock on them and if they sound hollow it is ripe.

This link tells you how to tell if a watermelon is ripe if it's on the vine or at the store.

https://www.treehugger.com/green-food/how-tell-if-watermelon-ripe.html

Same as Carroll ; thump on them lightly they'll sound hollow; here are 3 additional ways https://commonsensehome.com/tell-watermelon-is-ripe

It needs to be bigger and older because it is not ripe yet. That’s a lovely melon, but wait until the stem coming out of the melon turns brown before picking the melon. “Picking” involves just tearing the stem off and taking the watermelon to the refrigerator. Jewell