Pre-requirements

This should not be necessary in most cases, as Windows 10 has sideloading enabled by default

• Go to Settings > Update & Security > For Developers
• Select "Sideload apps"

Instructions

App packages can be installed simply by double clicking the app package file, just like you would with a traditional program's installer file. To use this, simply navigate to your app package (.appx) or app bundle (.appxbundle) file, and tap it.

Windows will ask if you want to install the app: choose yes and give it some time. It can take 20 seconds or more for the app to install and show up in the app list and there's no indication of progress.