The most basic scarf for joining two timbers together is the basic half-lap joint.
One timber has half its depth cutaway on the top, and one timber has half its depth cutaway on the bottom, and these two pieces lap over each other.
This is an exploded view of a simple half-lap of two 8×8 sill timbers.
When timbers are directly supported with a foundation, then this type of joint can be used.
It is not the strongest, just the simplest to cut.
Where would the post go under this joint ?
No where. Posts should not go under a scarf joint.
Jim Rogers, My brother, who builds timber frame barns, just told me to put scarf joints over posts. I need a 26′ 6×6 beam, with two posts (so, two 8 footers and a 10) how would you make that 26 beam and hold it up with two middle posts?
I’m sure your brother was doing what he thought was right. But all my teachers and timber framing engineers I work with say scarfs should not be over posts. Get longer timbers so you can scarf them beside the post over a brace to hold it up.