First you have to find the stones. Round, flat stones work the best.
Traditionally, when you take anything from the earth you leave an offering behind.
I found that a small paint brush was easier to control.
You should use the same colour (a dark colour is best, but you should use whichever you feel is right) for each rune.
Remember, one rune per stone.
When the paint dries apply a coat of varnish to the stone to protect it from wear and tear.
You'll need to reference runes for accuracy.
You can check out this site for that info.