Instant coffee is "just" freeze dried coffee. You make "normal" coffee, and then freeze dry it (and then usually you process the results to get nice shaped lumps instead of randomly sized flakes and powder). I say "normal", because you will get flavor loss/changes in the freeze-dry process, so you'll want to brew with that in mind.

And if you're interested in an actually (relatively) decent instant decaf, I'd suggest trying Mount Hagen brand of instant decaf coffee. Although, just from personal anecdotal experience, I believe it could have slightly more caffeine than some other decaf coffees.