Pruning montana types is easy, and hard to get wrong. Lightly prune after flowering each year to sort out the shape, and if need a very hard pruning can be done in early Spring - montana will regenerate from old timber very successfully so hard pruning is ideal. - indeed you could take a montana down from 20' to 3' very happily with a good saw!
For planting - Always plant Clematis montana about 5inches deeper than the soil level in the pot you buy it, this helps to keep the roots cool and prevent scorching of the roots. - and remember to plant at an angle facing towards the structure you want to cover....
For more on Clematis montana, check out the Clematis montana National Collection