Step 1: Find some Valentine's Day-appropriate construction paper. I guess thinner paper would do, but construction paper is nice and sturdy. We did ours with red paper, because that is what I found in our supply cabinet. Fold the paper in half.
Step 2: Hold your kid's hand on the paper so that the index finger and thumb are on the folded edge. Explain to them that they are going to make someone's day so much better as you do your best to trace their hand with a pencil. Don't hold on too tight - this is supposed to be fun. I imagine, if your child is older, they could do this him/her-self.
Don't worry about tracing the hand for each card. If you have a little one (like my 2 1/2 year old), send them on their way to play or, better yet, go play with them and do the rest later, when they're asleep.