To add a person to a deed you must prepare a new deed with all current owners. Then you must insert the legal description of the property. All current owners must sign the deed with a witness and notary.
The deed must be recorded in the office of the clerk or register of the county where the property is located.
I strongly recommend that you have this handled by a real estate attorney or title agency. It shouldn't be expensive and this way you can make sure that it's done properly. If you do it yourself and make even a little mistake then it can be very difficult and costly to fix.
You should also be aware that, if there is a mortgage on the property, then it is very likely that the filing of any type of deed will be a violation of the terms of the mortgage. You may need to get written permission from the lender to do this type of transfer.
Also, there may be tax implications as a result of the transfer.