Pruning shrub rose as a climber

I got a Gertrude Jeykll shrub rose in July which is planted in a deep barrel and want to prune it to make a small climber which it can be used as. It has produced one tall stem at the back but am unsure what to do now. I know climbers need a pruning regime so they don't have all their flowers at the top, so any advice would be welcome.