The first picture is the finished joint to accept the beam. No power tools here, cut with a hand saw standing on a step ladder, be safe tie the ladder off.
Stagger your joints.
All finished, two down and one to go.
A future project will be to add a steel channel on each side of the beam. Bolted through the posts and beam it will receive a hoist to do vehicle and equipment maintenance, no more jacks, and it will be easy to unload heavy items off my truck.