Take the fill off.
Use pathfinder, divide button.
This groups the shapes.
To make something shorter, select with white arrow - only the points you want to move - then use the arrow keys to move them around.
collar shape has to be at the front, after you have drawn the ribbed brush over this you have to move the brush to the back. Similar to divide, select both.
Clipping mask - object, down the bottom is clipping mask
Right click, make clipping mask. This covers up all the area outside of this shape. Can use white arrow to change this.
Now we cut clipping mask out and put it back in the group
Select, apple x, go back into group and control v. Move it so it fits on top.
We can always go back into group or back into mask.
Make a back collar.
Bear in mind that once we have made a collar once we never have to do it again. Put it into our lovely symbol library and never ever draw it again.
One thing missing on t-shirt - back of shirt - just make any old shape and send to the back.