Dale Arends says:

First off, wash them thoroughly with 70% isopropyl alcohol to remove any oils and residue. Then spray them with a coating of dull coat so that any weathering you apply will stick. next sand them lightly. Depending on how dark they are, apply some light gray or tan powders or chalk until you get the effect you want. Depending on the type of logs you want them to represent, you may want to dust them with a bit of color but don’t overdo it. A little goes a long way. When they are right, hit them with the dull coat again to protect the work you put into them.

Weathering is always trial and error. If you like the results, you did it right. If not, clean them off and start over.