If you want to use the SharePoint Sandbox solution to install the Web Part(s) on your website, then you are in the right place. 😉
Open the settings menu (⚙) and click on Site settings;
Under Web Designer Galleries, click on Solutions;
On the tool bar click Upload Solution and select the installation .wsp file (eg. BTXXX_x.x.x.x.wsp);
As soon as the upload is complete, click on Activate and wait for the page to refresh.
Web Part installed! ✅
Done! To complete the setup process the only thing left to do is to add the Web Part to the page and proceed with its configuration.