Sweet peas self pollinate, so they have already pollinated if the flowers are open. To cross sweet peas, you need to open the flowers when they about ready to but not yet open, then take the pollen from an already open flower and manually pollinate them.

The book "Breeding New Plants and Flowers" by Charles W. Welch has a cahpter of sweet pea hybridizing. See if you can get it through interlibrary loan. It is a popular book.