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.
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.