pratcise, practice. practice and dont fool you self by going for the slightly bigger ones. the larva you can bareiy see are the ones you need to graft.
i did 60 grafts into 3 colonies in a litte over 30 minutes this eve.
At the risk of going off at a tangent the colour of the wax on queen cells is supposed to say something. Pale wax is newly made by the bees and is a good sign they were keen to raise that particular new queen. Dark wax is recycled and is less of a good sign. I don't think I have the experience to refute this or not but the different colour of the wax between cells in the photo above is marked.
the difference in the colour of the wax was that there was a honey flow on and they started to incase some of the cell in comb and i had to cut them out inorder for them to fit into their queen protecter cages