I count the number of HSTs I need, count how many in each color combo...we'll say i need 100 for a whole quilt in black an white. I draw a grid on the back of the 'light' fabric...it could be 10 x 10, 2 x 50, 4 x 25, whatever fits (that equals 100) on my fabric because this is determined by the size of the square....
Now what do I need? Finished size = 3"... so I draw a 4" grid. You do not sew on these lines, they are cutting lines for later. Do NOT cut now! Draw one set of diagonal lines from corner to corner starting in the corner square and then moving to the next diagonal..all the way across the grid... You only do this one direction for HSTs. Pin the fabric together, avoiding diagonal lines and placing no more than 6" apart. Beginning in the center, sew a 1/4" seam on BOTH sides of the longest diagonal. Repeat in both directions from the center outward, sewing on both sides of ONLY the DIAGONALS. Press the entire grid so that your stitches are all set in one move.
Cutting: Cut the grid apart on EACH DIAGONAL line. Now cut each strip apart on the VERTICAL AND HORIZONTAL lines....you now have 100 seamed, pressed HSTs. Begin to open each folded square with the dark half on top. This will make it easier to press the seam to the dark side. I just open another square on top and shift it a bit, ironing both, over and over till I have 6 or 8, then move the stack and begin another. When all your pressing is done. Collect the squares, a work surface (kitchen cutting board works well), a small cutting mat and a 3.5" square ruler and rotary cutter. Put a chick-flick in the TV and begin to square up your little squares, matching your seam to the center diagonal line on the ruler. Cut 2 sides, turn 180 deg and trim the other 2 sides. you do NOT move ruler or square, rotate the small mat on the bread board... nothing moves or slips this way... line up right ON that bias seam and do not move... there will only be slivers, that's normal....
If you need an hourglass, the only difference is that you allow an extra 1/2", these would be 4.5" and then mark your grid with TWO diagonals. Everything else is the same till you get to the 'open and press' stage. Open, press and then flip to stitch the opposite diagonal line. It will only go halfway so you may want to draw the other half. If i have the first half, I feel secure enough to go across the other half with a mark on my sewing machine where the point of each square is lined up...Line on first half, follow point on the second half. Cut the cut the squares apart and do the whole open and press and square up, but this time match the center of the ruler to the 'center of the X.' You want the block to be symetrical...it has to be centered to achieve this. The only differences here are the extra 1/2", the 2 diagonals, matching the two squares together (be sure to match light to dark, on both sides), and then the second press, cut, open and square up after (not 2 square ups...only after the second half). When making a large number of squares, I put one extra line on the grid...that way, if I mess up a seam or the cutter slips, there are a few extra squares to fill in without any extra sewing.
Why do I allow the extra bit when drawing a grid? Because we do not think in 7/8" ... it is way faster to put that entire inch on the finished size, faster to draw and figure.... then I have a bit to square up and so each HST is perfect because it was squared up AFTER the sewing and pressing... if you make your HSTs with the 7/8" extra, you have to rely on perfect cutting, perfect sewing and non-stretching pressing to achieve this and it is difficult to measure and cut at the weird measurement. I am saving so much fabric with the grid method that the teeny bits I trim off are well worth it for the accuracy I get. Make a small sample, 2 squares x 2 squares... (2 x 2 = 4 to sew, but you will get 8 HSTs from this...as there are 2 HSTs per square, finished)