To create two HST units you will begin with two equally sized squares of fabric. You will want to cut these original squares .25" larger than you want your completed HST units. The next step is to draw diagonal lines on one of your two squares (I would choose the lighter fabric). I prefer to use my Quick Quarter ruler for this step. You are able to line the center line on the ruler with the diagonal and then draw a line on either side of the ruler. The ruler allows for a one-quarter inch seam allowance on either side of the diagonal - this may make more sense after seeing further steps.
You will sew along each of the outside lines. Turn your machine speed to slow so you have control over the stitches and can make sure you are moving the fabric through the machine in a straight line. You can choose to pin the two layers together for this step, or forgo the pins if you prefer.
Once you have sewn both of the lines you will cut down the middle to create two triangles.
Press each of the seams to create two squares. I choose to press my seams open.
Lastly, you will want to square-up your blocks. To do this, align the diagonal line on the fabric with the diagonal line on your ruler. Trim the sides of the square to the correct square size you need for your pattern. It is really easy to do this if your template is the same size you need - but it can be larger as well.
Congratulations on your HSTs!! Once you get this skill down there are so many patterns you can make with these units. Take some time to play with layout - you will be so excited by the things you can create!