You can have guardianship papers drawn up by an attorney for a small fee. We had ours added to our will/estate/trust paperwork right after my son was born. It is a fairly simply document and will name the guardian, anything else you will in care for your children, etc. (for example we named by parents and made my father executor of our estate. Everything is left to our son, in trust, for my parents to use as necessary) You can specify if you want money used for certain things, i.e. college, etc. You should also name a contingent guardian, just in case.
I would actually contact your local court and ask if they have the paperwork. You may be able to simply fill out a form and have it notarized. Make sure your guardian has a copy!
Most law firms would perform this servce for a small fee (probably less than 200$).