Its a relatively simple process. The family that is having the child baptized simply needs to inform the priest that one of the Godparents cannot attend and will need a proxy. You will submit your "sponsor form" as usual (the form which attests you are a practicing Catholic in good standing). The person who will stand in as a proxy will do the same. At the baptism the proxy will stand in your place and act on your behalf. In the baptismal registry you will be listed as the Godparent and under your name will be the proxy's name with a notation that he or she acted as your proxy.