" How would you go about forming these slots? *What sort of tool?

Buy the finished poles at the Home Depot or Lowes. I believe they
call it a split rail fence and sell the posts and the fence boards,
too.

But, as many woodworking enthusiasts, my knowledge of fencing in
livestock is somewhat limited. Thus, I would suggest you might Google
"livestock fencing installers" to get an idea of what works before
"tooling up."