
If they have finishe flowering, then cut back a third in the Autumn, Sept/oct so the snow and wind doesnt break them over the winter, and the 'surplus' protects tham from the frost, then in March time, cut back to 3 leaf nodes(where the leaves sprout from) from the bottom.
BUT, if they are old roses you have to be a bit more careful, and it is harder to explain, so if they are new, do what I have said, If they are older tell me and I will explain more LOL
HTH
Lea :)