It sounds like they are having you make half square triangle units (HST's). When I make HST's, I cut them 1 inch larger than the finished size required instead of the 7/8" they are having you do. I then place 2 of those size different colors, right sides of fabric together. Draw a diagonal line, sew 1/4" on each side of that line. Then slice on the line to get 2 sub units of the different colors. After pressing, I trim them down to the proper finished size. Yes, this makes extra trimming/cutting, but it ends in more accurate units for me.
In theory, if you cut the 3 7/8" units accurately and sew them accurately you would have no need to trim. I'm not that accurate in either case, so I up size accordingly.
Check your directions to make sure these are the units they are actually having you make. I find it helps me to read through the entire set of directions first before tackling any cutting. You could also do a test with paper first if you have no extra/scrap fabric to make a test block.