You shouldn't need to encourage it to trail - if there's nothing for it to climb up, it will cascade down, fixing itself to the wall as it goes. You might need to rearrange the stems to spread them out a bit as it starts to grow, and you should work in plenty of organic material such as composted animal manure to the soil in the border where you'll be planting. Bear in mind it wants to get more than 25 metres in height - you will likely need to prune it back fairly hard in winter as it matures to keep it in check. Always prune (if necessary) only from autumn through winter, because this plant is prone to excessive bleeding if cut at other times, though removing the odd stray stem won't cause a problem.