Train the two bottom shoots on either side of the leader to garden stakes secured with ties to the horizontal supports at a 45-degree angle from the central one (training first-year branches at a 90-degree angle retards growth).
In autumn, as the leaves fall and the tree goes dormant, remove the stakes and lower the two side branches to 90 degrees. Tie them directly onto the bottom tier of the horizontal supports. Then, prune back the leader to within seven or so centimetres of the top of the second tier, again making sure there are three good strong buds at the top, which will develop into the central leader and two new horizontal arms in Year 2.
At this time, lateral branches (the small, twiggy shoots that have emerged from both the leader and the arms of the tree) should also be pruned to the healthiest three buds (or three leaves if they're still present) closest to the next set of horizontal supports. These twigs will then develop into flowering or fruiting spurs.
Prune the arms of the tree by about one-third their length, cutting just above a downward-facing bud.