This will seem very basic if you've owned a French easel for a long time, but can you save you some guess work if you just acquired one.
First extend the back leg which is folded under the box of the easel.
Then comes the tricky part. I balance the box on my knee and hold one side of the box with my hand.
I unfold one the leg opposite to the side I am holding with my hand.
Standing on two legs, the easel is more stable and I don't need the support of my knee any more. I can now unfold the third leg.
I make sure the box is level and adjust the extension of the leg if necessary (if the surface is bumpy).
Now I can open the box and get ready to paint.