I've just recently discovered the easy way. Use Thangles. Example: If I want my finished block to be 2" then I use 2 1/2" Thangles, so as for a half block, then Open, Press and match the two right sides together, dark to light. Be sure the seam on the 1/2 block is opposite the seam on the thangle for the bias rectangle. Line up the diagonal edges so the Thangle sewing line is exactly on the two oppsosing corners, then sew on the line that is on the two corners. Press, tear off paper and Whala there is your 1/2 bias rectangle.
Hope this wasn't too complicated. You might try the Thangles web site, I think they have one it might be clearer on the instructions than I gave. :?: