I know about 8 methods for making HST's. Some of them involve special rulers, specially marked papers, or other tools to simplify the process. I have my two favorites (a tool and a paper product), but I can and do use others--they each have their pro's and con's.
This tutorial will show the most common method for making TWO HST units.
finished size of HST unit + 7/8"
Cut two squares of fabric this size (coordinating fabrics, contrasting fabrics, or focus fabric + background fabric). For example, a 2" finished block will use two squares that are 2 7/8" by 2 7/8". This measurement allows for the diagonal seam allowance, and will result in TWO HST units that each measure 2 1/2" on a side (and will finish within a larger quilt block at the desired 2")
On the wrong side of one of the fabric squares (choose the lightest fabric for easier drawing), draw a line along ONE diagonal.
You may use any straight edge/ruler for making this line. Similarly, you may use any pen/pencil that will make a narrow line; the line will be within your seam allowance and will not be seen.
Alternatively, a tool called the Quilter's Rule Quick Quarter can be used for this marking step. If you use this tool (very brief video here from Connecting Threads on YouTube), you will mark not only on the center of the diagonal, but along each of the edges of the tool. This marking should be done in pencil.
Place your two fabric squares right sides together, pinning if you wish. (I pin here because I am cautious when sewing along a bias, and don't want any chance of distortion.)
Stitch 1/4" away from each side of the line drawn along your diagonal.
If you used the Quick Quarter tool, these 1/4" stitching lines have been drawn by you already. Simply stitch along the drawn outside lines.
3. Cut and press
Cut apart your square along the center diagonal line. Open the two HST units, pressing the seam allowance to one side (usually toward the darker fabric, unless your pattern advises otherwise).
Want more HST techniques? I have written about a couple other techniques here and here (these were not meant to be tutorials, but do include some details)
Can you learn a technique from just pictures and text, or do you need a video tutorial?