I am building a raised Garden box using the landscaping timbers. I am using 4 layers of timbers and am dovetailing the corners. My problem is the rebar are not going through the hole into the next timber. I think my problem is the timbers are shifting and the holes are going out of aliment. So what is the right technique to do this? Do two layers at a time? Have someone else hold the timbers so they don't shift? When I did a box with only 2 layers I had no problems hammering the rebar through the timber holes.

Rebar isn't always straight. You might consider a slightly bigger hole. The rebar ends should be radiused like a spike.

You are over-thinking the application. You should only have to stagger the timbers. No one I know uses dovetailing or rebar. How high are you making the boxes?

4 rows of timbers high. I found that putting my foot on the timbers stop them from shifting as I hit the rebar in as well has not hitting it hard allowed me to get it though all 4 timbers without problems.