I lost my iphone X , it is paired with my apple watch. I bought a new i phone 12, how can i pair it with my new phone?

How to erase your Apple Watch if you don't have your iPhone

1. On your Apple Watch, tap Settings > General > Reset > Erase All Content and Settings. 

2.Type your password if prompted.

3.For GPS + Cellular models, choose to keep or remove your cellular plan.

• • If you want to pair your Apple Watch and iPhone again, keep your plan. 
  • If you don't want to pair your Apple Watch and iPhone again, remove your plan. If you aren't going to pair with a different watch or iPhone, you might need to contact your carrier to cancel your cellular subscription. 

4.Tap Erase All to confirm. This resets your Apple Watch to the factory settings.

For more info, see here: https://support.apple.com/en-us/HT204568

To unpair and erase watch and remove Activation Lock.

On a computer, go to iCloud.com and sign in with your Apple ID.

Go to Find Iphone.

Select All Devices, then click your Apple Watch.

Click Erase Apple Watch.

Select Next until the device is erased.

Click the remove button next to your Apple Watch.

Then go here;

Pair the watch with my new phone.