Start with cutting your squares. 2 squares of different fabrics make 2 half square triangles. You want to cut your squares 1/2" larger than you want your unfinished half square triangle or 1" larger than your finished half square triangle. (same thing just depends on how you like to calculate your sizes.) So in this example, I will be making 2" finished half square triangles so I will cut my squares 3" x 3". Also, you are almost always making half square triangles in a mass quantity so cut all of your squares to start and then to save time be sure and repeat each of the sub-steps for making your HST's in an assembly line fashion.
On the wrong side of the darker fabric using your ruler, draw a pen line diagonally from the top left corner to the bottom right corner.
Lay your dark square on your light square with rights sides together. Pin both sides of each square adjacent to that drawn line. Make sure to put them out far enough that your presser foot won't hit them as you sew on by :) Now stitch a 1/4" seam along both sides of the drawn line (the black lines represent your stitching lines :) Remember, you are doing these assembly line so go ahead and chain stitch all the sets on the right hand side of the line, and then go back and do all the sets on the left hand side.
Clip your threads and then align your ruler along the drawn center line. Cut your pieces apart along that drawn line.
Now press towards your darker print.
And now you are ready to trim up your HST. You will trim to your pre-decided unfinished size. (1/2 smaller than you started with) In this example I am trimming to 2 1/2". Start by aligning the 45 degree line on your square ruler with the seam line on your half square triangle. Make sure that the raw edges of the half square triangle are over the 2 1/2" line's on your ruler. Trim the 2 sides along the edge of the ruler.
Now spin your block 180 degrees. Once again, align the 45 degree line on your ruler with the seam line on your half square triangle. Align the raw edges of the half square triangle under the ruler on the 2 1/2" lines on your ruler. Trim the 2 sides along the edge of the ruler.
And now you have Perfect Half Square Triangles. They take a bit to trim but are totally worth it for perfect points!!
If you don't have a square ruler you can use your long ruler to trim like so . . .
Once again, You will be trimming your half square triangle to a 1/2" smaller square than you started with. So, since this started as a 5" square, I will be trimming it to 4 1/2". To do this, align your ruler so that the 45 degree line is along the diagonal seam of your block and that the left side edge is on the 4 1/2" line of your ruler. Trim the excess on the right that is sticking out.
Rotate your block 90 degrees and align your ruler just like before. Trim away. Sometimes, there isn't a whole lot to trim away. Keep rotating your block until you have trimmed all 4 sides and have a perfect square :)
Now just start playing with the layouts . . . The possibilities are endless :)