If you can get help to keep the clamp from slipping off, all the better. Chisel the wood out as you go. The cedar is very soft. Since the overall speed of construction is quite fast, it is worth spending extra time on precision cuts like this. Close-up of the same.
Sanding block helps to knock down irregularities on the cut edge.
Think ahead how you want to bond the stem strips to the sheer strip. It is inefficient and messy to do it one strip at a time. The pre-gluing is just a clamp to hold the bundle together. One side is done first until the glue sets. It must be then shaved off so you can bond, bend and overlap the strips from the other side.