The longer the legs and A-spreader, the bigger your steps, marking with your A-frame, on the land will be.
Use some rope to wrap the top of your A-frame, binding the two legs together.
After that bind the horizontal stick on the two legs, same distance from the bottom up on each leg.
When you finish building your A-frame, make a mark where the rope with something heavy at the end, touches the horizontal stick. You do this on a horizontal floor, for example in your house somewhere. When the rope hangs exactly there, the legs of your A-frame will be at the same level.
Put a stick in the ground at each leg of the A-frame, when the rope is balancing at the mark.
My first swale boundary is marked with bamboo sticks on meadow 1 (zone 2+3).
Tip:
Probably using flat wood - instead of round wood - will be easier making your A-frame and could be measuring more secure.
Probably using flat wood - instead of round wood - will be easier making your A-frame and could be measuring more secure.