Open the Template, and also open one of your own maps. Now, go to the template and use click-and-drag to select whatever stuff you want. When it's got the white rectangle around it, press CTRL + C to copy it. Then go back to your own map and the copy of what you selected should appear when you move the arrow over the green stuff. Press CTRL + V to paste it.
When you're moving between the Template and your own map, make sure you click on your own map's window, not the body of the map itself, or the thing will be un-copied.
Also, press T in the Mapper to see the Types, which are self-explanatory.
To add enemies and platforms and junk, play the level by selecting it from Other Maps and press the E button to access Events Editor, and then hold down Right Click and press left, right, up and down to select whatever you want to add.
You're lucky you're getting this explained to you: I worked all this out myself Dx