If your monitors are from the same manufacturer and model, you can probably skip this section. Once you've connected both, Windows should automatically expand your desktop horizontally. Just set the stand of each monitor to be perfectly aligned and you're ready to go.
However, if you have two different monitors, you may need to work a bit more to make them play well together. Example: You connect your laptop to an external monitor and use it side by side, or you have a 4K monitor next to a 1080p monitor. This results in strange behaviors that are easy to fix: just right-click on the desktop and select Display Settings.
Under Choose and rearrange ads, you can click and drag the rectangles to match the orientation of the monitors on your desktop-say, one slightly lower than the other. In this way, moving the cursor to the left will bring the cursor to the same position on the left monitor instead of jumping up or down on the screen. You may need to do a little trial-and-error to get it right.
Scroll down to the Scaling and Layout section, and you can adjust the resolution of each monitor and its scale. So, if one monitor has 4K and the other has 1
If you want to go further, you can use the built-in settings of each monitor to match brightness and color as closely as possible. (An app like ScreenBright can make this a little easier if your monitor supports software controls.) Once you've made all these settings, your monitors should be much better matched so that moving windows between them will be easier and more enjoyable.