Iwatch series 6 install sim card

There are two versions of Apple Watch. GPS only and GPS + Cellular.

Apple Watches, which are cellular, don't use physical sim cards. They use eSim technology. If your watch is actually cellular, you can tell by looking at the digital crown and if it has a red circle on it, it is cellular. If it doesn't have a red circle, it isn't cellular.

Assuming you own a cellular Apple Watch, this link will tell you how to set up cellular service on the watch --> Set up cellular on Apple Watch - Apple Support

Note, you must use the same carrier you use on your iPhone. AND, that carrier must support Apple Watch. To know if your carrier supports Apple Watch, this link identifies every carrier around the world who does --> Apple Watch - Carriers - Apple

How on iwatch do I insert the eSIM? This is a Apple Watch Series 6. Please help me :)

eSim is electronic and you don't insert an electronic sim to any device. Contact your carrier to set up cellular service on your watch.