Hi DiMiTriX, i attach some pics for you to see how i am trying to graft areoles on a opuntia pad. I am no specialist it is also my second time i'm grafting something.. but practice makes perfect so they say.

What i gave attention to, was that the areoles should be on top of the center and also on top of the more greenish border. The same applies for your seedlings.

The first pic is Trichocereus Bridgesii Monstrose areoles grafted on opuntia pad.

The second pic is Trichocereus Pachanoi Crest showing the rib i removed.

The third pic is Trichocereus Pachanoi Crest areoles grafted on opuntia pad.

Tomorrow or so i will attempt to make a longer pachanoi rib grafted on opuntia like i pictured in the first post of this thread.