I do not know how to answer your question with one answer so I will break it down into parts.
All formal invitations are similar in look and content. On appropriate paper, either write or have printed the invitation to the baptism. You can follow the outline for a wedding invitation substituting your information.
If you are using the invitation as the means of asking the invitees to be godparents, then you can just add that line. However, mixing the invitation with an important proposal of this nature is not good form. Remember, it is far more acceptable for the "potential godparents" to be approached in person prior to the baptism on an topic of this importance.
And it is never good etiquette to surprise one in front of others at a formal or important affair such as a baptism. Good manners dictates that we strive to avoid any situations that cause awkwardness or embarrassment to others.