please help! bought a cool iron on...now what?

[pjmjsj]

Hi there- I wanted to get my family matching shirts for pirate night on our disney cruise. I found a great image on etsy and the seller was really nice and put it on transfer paper (reversed image) for me. She even cut it out for me. There's four of them- two larger for me and DH and two smaller for the kids. They are beautiful but am concerned that I will mess the transfer onto the shirts. I have never put an iron-on on a shirt before. I just want to put them on plain white, or maybe black 100% cotton shirts. As is very very clear, I don't have a crafty bone in my body. Can anyone let me know how it should be done? thank you so much!