If you cut the sashing the same length as your blocks:
Put the sashing on top of the block when you are sewing it to the block.
Ease the block to fit the sashing.
A little bit of fullness can be pressed out. (if it's only a little bit)
Machines without a walking foot tend to feet the bottom layer a bit faster than the top layer/
You aren't dumb - it's hard to put into words what is relatively easy to show by example.