How you trim a plant will depend on the plant. If you can give us a list of plants you have we will be able to better tell you how to trim them and or replant the trimmings. If you don't know the name a pic of the plant(s) would work too.
For that one all ya gotta do is pick a spot on the stem and cut. Then replant the cut part in the gravel and you'll have a new plant. It really is that simple...lol