Enable Developer Options
On your Wear OS watch you’ll need to go into Settings, System, About, find the Build Number and tap this seven times. You’ll now see a new option in the Settings menu called Developer Options.
Turn on ADB debugging over Wi-Fi
In the Developer Options menu look for and enable an option called ADB debugging, and confirm that you wish to do so when asked. Below this you should also see Debug over Wi-Fi. Enable this, give it a moment to connect, then jot down the IP address that appears below. It will look something like 192.168.0.27:5555.
Install Minimal ADB and Fastboot
This is how you install it on a Debian based linux. sudo apt install adb -y For other systems, just look online, you should be able to find it for yours.
Use a website like apkmirror to download the APK file you want to sideload.
Rename the APK
Make sure you give it a simple name like hangouts.apk
Connect Minimal ADB and Fastboot to Android Wear
In terminal type adb connect 192.168.0.27:5555 make sure you replace the IP and port with your smartwatches IP and port number.
Install the app on your watch
Open the terminal emulator and use the cd or cd .. to browse to the location of the apk you downloaded. After doing so, run the command adb -e install [name of file].apk Make sure to rename “[name of file]” to your file name.
Wait and finish
You now need to wait one to three minutes for the file to finish installing. It will seem like it is doing nothing, and then it will say success.
An app you can sideload, that works, and was (for some unknown reason) removed after an update, is hangouts. You can download it from HERE.
Remember: You can use hangouts with google voice, if you configure a Google voice number and configure hangouts to use that number in the settings. This is good as if you want a separate number for texting to your wear OS device, you can do so,