If you have a vsix file, you can install it by running the file.

1. Get the vsix file (this is the extension installer)
2. Run the file.
3. In the window that opens, confirm the installation.

In Visual studio

• go to Tools > Extensions and updates...
• In the window that opens go to online
• Select Visual Studio Gallery
• You can search for an extension on the search box at the upper right corner
• Select the extension you want to add
• Click on download.
• Once download is complete, click on the Install button on the window that opened.
• In order to use the extension, you might be requested to restart visual studio