ViSu, You reminded me of the same experience I and my friends had before. In our college days, we planted a number of saplings on a barren land\hill slope.
As you are going to plant in a hilly area (which could be really hot these days), you need to take care of few things:
Ensure your saplings are not very small. (Small plants can be eaten by local animals)
You have to regularly water them for at least a month. (We planted many saplings, but we didn't water them, and at last all plants died. We had to plant them again.)
If possible, give them fertilizer (dried manure is also fine)
(Only if required) use a pesticide to keep insects under control (diluted neem oil spray).
If possible, guard the saplings physically, such as with mesh protectors or vinyl tree guards.