The follower or priest must activate the statuette of their God and right click on you, selecting convert under the religion tab. You must have toggled /invitations for this option to be available. You may only be converted once every 7 days, and must have a body skill of 1.5 or above to get converted by another player. You can't be converted if you're mounted on something like a [[horse]] or [[cart]].
*When being converted by a Priest your starting Preaching faith will be equal to 1/5th of hispreaching.
NOTE: It was previously stated you need 1 alignment to convert - This has been tested and proven false.