I want to add my friend to my mortgage loan. How do I do this?

Why,

You already have a mortgage? Why are you wanting to add someone to the loan.

And even better why would they want to be liable for your mortgage?

In general you will need to contact your loan company and see if someone can be added, most likely they will want to re-write the loan to add the mew person, but it is all dependent upon the terms of your loan,

Contact the loan company and ask them.