I use 1/4 "seam and trim them after I sew the basic pieces together.
example: after I sew a half square triangle I will press it and then trim the seam allowance.
If I am making a four patch I will trim the seam after I sew the first set and after the second set. Then I will starch and press the seams and the trimmed patches are ready to sew into a four patch.
Each seam is trimmed as you go and then the entire block is ready to piece and all the seams are trimmed and ready to go.
I do the same thing as I sew the blocks together.
Sewing an 1/8" seam is almost impossible to get accurate.