Once you've added the SSI tags to your pages, you need to upload a simple file called an htaccess file.

An htaccess file is just a small text file that gives special instructions to the server your pages are hosted on.

In this case, we want to tell the server to look for SSI tags.

htaccess

Create a simple text file and name it:
.htaccess (Don't forget the dot!)

Paste this single line of text in it:

AddHandler server-parsed .html

And upload it to the root of your domain.

Don't forget it may "disapear" on your server after it's uploaded.

That's it...