It's not hard to do, but you'd better know what you're doing.
1. Can you read a schematic? It helps to know how you're going to wire them.
2. Do you know how to solder? If not, you'll make a mess and possibly damage the pickups, the switches and/or the pots.
3. Are you any good with a router? Humbucks won't fit in the same cutouts as single coil strat pickups. Side by side coils are wider, and stacked ones are deeper.
If you're OK with all of these questions, I'd still ask why you'd want to screw up a perfectly good strat to install humbucks. I prefer humbucks, but a strat is not designed for them. Get a guitar with a body that is, and changing pickups will be far easier. There are some Strat style bodies that are already routed for humbucks. You can find them on "build your own" custom guitar sites, as well as from a number of regular manufacturers.
Screwing up a real American Fender Strat is a bad idea.