I would make them no more than the day before I need them. You don't use anything to attach the fondant. You have the fondant cut out and waiting. As soon as you pull the cookies out of the oven, lay the fondant shape on top. The heat of the cookie will laminate the fondant onto the cookie. Work fast!
They will keep for a week at least, individually bagged.
Leah is right about how to attach the fondant. I cut out all the shapes in advance and lay them out on wax paper dusted with cornstarch. Don't stack them. They'll start to dry a bit and that helps them keep their shape when you pick them up. Put them on the cookies right after they come out of the oven, then let them set on the cookie sheet for a couple minutes. Don't touch the fondant until it cools, or it will wrinkle. If you're using an impression mat, you'll have to figure out when the fondant is cool enough that it's not sticky, but not so hard that it won't take details...for me that's about 20 minutes after the cookies come out of the oven.
Some people use corn syrup to attach the fondant, but the heat method is much faster and produces better results. Within an hour the fondant is firm enough that you can stack the cookies, useful if you are making a lot of cookies or don't have a huge kitchen 
I use buttercream to attach fondant to cookies. I spread a very thin layer of the buttercream on the cookie then place the fondant on top. It's worked great every time!
If you want to make your cookies further in advance, you can always freeze them. I've made fondant covered sugar cookies with royal icing details, wrapped them individually in cellophane bags, then put them in the freezer. I had them in there for just over a week. The following day when I tried them, they tasted just as fresh as the day they were made! Just make sure you give your cookies time to fully dry before putting them in the freezer.
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