A lot of things sold as "fans" are really a mess (I stormed out of a Wyvales last week over this!), with the main leader growing straight up and strong (and branching higher up too), and the trained-in side shoots at the bottom much weaker. Properly, you have no main central stem above the first two branches, and all the other branches spread out from one of these side bits (kind of like a capital Y shape with all the other branches fanning out from the tops of the Y). This keeps all the branches growing roughly equally, rather than lots of strong growth in the middle and weedy stuff at the sides.
You need a young tree that's got a pair of strong branches (or good buds) near the base, growing in roughly opposite directions at about the same height (a slight difference doesn't matter, but more than a hand's-breadth will start to be annoyingly lopsided; also, "roughly opposite" probably translates to more than a right-angle between them!)
First step is the hardest: lop off the whole top of the tree just above your chosen two branches. Then cut back the two branches to under a couple of feet (depending on how big a fan, maybe as short as one foot) to encourage them to branch. If they're just buds, you may have to wait for another year before continuing, unless they put a lot of growth on very quickly! Tie them into canes pointing a bit up of sideways (um, maybe 30-40 degrees above horizontal) that you've tied to your support wires (ie tie the canes into the right place where you want the branches, then tie the branches to the canes not the wires.) Rub out any other weedy shoots below them with your fingers. Do this soon after they break bud in the spring: about now in Cornwall, but maybe early May up north. If one side is growing stronger than the other, untie the canes from the wires, and raise the weedy side and lower the vigorous side. Once they're matching up again, move them back to the same angle.
In midsummer, once they're growing well, choose two branches on the top side of each side-branch, and one on the bottom. You want nice, sturdy shoots that are evenly spaced and pointing fairly parallel to the wall. Leave them, and one near the cut-off end, but rub out all the rest. Tie them into canes at nice evenly spaced angles to fill in the gaps (and the one at the end to carry on the line of the branch you cut back).