I've never used muslin or paper as foundations for log cabins. Some hints:
1- press and square up the blocks after adding the first 4 strips to the center, and again after the next 4, etc.,
2 - make the last 2 strips (a light and a dark) of each block 1/4" wider than the rest. So if your strips are 3" the last 2 will be c ut 3 1/4 each.
3- after pressing square up the block to it's final size.
Hope this is helpful. May in Jersey