Download the GApps package you want to install.

Repeat step 10 to reenable ADB sideload.

Enter the command adb sideload [directory]\[filenameOfTheAppsPackage].zip

If the script tells you that the API level does not match even though you are using the correct GApps package for your version, you can typically resolve this by rebooting into recovery.

Do not boot into the system before flashing all the extra packages you want to install.

Some recoveries might tell you that verification failed. You can ignore this and continue anyways.

For some devices you might run into space problems. You can typically resolve these problems by choosing a smaller GApps package and later installing what you need through the Play Store.