What you'll need:
Well, duh. Can you use rocks you find in your yard? These probably aren't smooth enough to do well. We suggest buying a bag of smooth rocks. You'd think you could find them easily at your local building supply or hardware store. We haven't had great luck finding them. See bags like this one.
Literally where the paint hits the rocks. A set with a variety of brush widths is good. If thery're all too thin, it'll take you forever to paint the sky.
After the paint's dry, you'll need to seal it against the rain and elements. Spray sealers are quick, convenient, and don't create a mess to clean up. Be sure to use this stuff outside, as the fumes will get to you.
Optional, but nice to have:
You really don't want to dip your brush into different color paints, you'll mess your paint up. You can pour paint onto a smooth surface, but then it might run off onto your table. These little palettes are cheap and keep your puddles of paint orgainzed. They pay for themselves quickly by preventing wasted paint.
Especially fine-tipped ones like used in nail salons. These can be used for delicate shapes or just lots of fun dots.
Paint Marker Pens
Many people find they do better fine work with a pen rather than a brush. We're used to writing with pens, so it's probably easier to control. Depending on your tastes, you can go with fine-pointed markers or extra-fine pointed.
A lot of people print out a note on the bottom of their rocks asking the finder to post their find to a facebook group with a certain hashtag. These paper notes are often stuck to the bottom of the rock using Mod Podge, an adhesive well-known and well-loved in arts and crafts circles. Just remember to stick the note to the bottom and let it dry thoroughly before you seal your rock.