If you want to assign a splash page to a specific location, you can click Manage → Locations in the left menu toolbar, double-click the desired location, move to the Settings tab, click Edit and change the Splash Page value from the drop-down menu.
In some cases, however, you may want to assign the Splash Page to multiple locations, and you need a faster way to do the job. A suitable solution for you is to click Manage → Splash Pages in the left menu toolbar, and click on Apply to () in the row corresponding to the splash page that you want to apply to your locations.
In the subsequent screen, just select your locations (in the Locations input field). If you enter an existing tag in the Filter by tags input field, then all the locations with that tag will be added.