TWRP is a touch based recovery and thus may be very difficult to use if you have large Hodor hands considering the size of the displays on both the G Watch and the Gear Live. If you have a stylus handy I would suggest you use it to minimise any errant touches. Compounding this small screen is the fact that the recovery doesn’t seem to take up the entire screen so once this issue is sorted then it may be easier to use. Until then proceed with caution.
There are two ways to install TWRP, via an app from the Play Store or via fastboot using the images from the relevant TWRP pages for each device. The G Watch page and the Gear Live page both have links to download the image file required for this fastboot process. To install, once in the bootloader screen and you have determined your smartwatch is recognised perform the following command:
fastboot flash recovery path_to_recovery_image_file.img
As always read about what you are doing and follow any and all instructions to the letter. Flashing a wrong file in place of the recovery may result in bricking your new smartwatch so a cautious approach is highly recommended.