Once again...go to the bottom picture in this post and work your way UP.
Because I want to keep my selvedge on my fabric still, I move the rulers to the end of the fabric and do this process here for the cutting. Once you have your cutting ruler lined up then rotary cut the bias. 
Then place the metal cutting ruler under the plastic square trying to line it up with the diagonal. Hold the plastic square over the metal ruler...make sure it's square with the selvedge...and use it to adjust the metal cutting ruler to the diagonal line.
Use any of the square rulers that have a line drawn from corner to corner. That line is the bias line. I hold my cutting metal ruler over the diagonal line...
Find the selvedge on the fabric you want to use.
To find the bias of the fabric you can take a square of fabric...44x44 inches and fold in half diagonally. The bias is along the edge the ruler is sitting on. If you are making smaller stems there is no need to cut your fabric in half.
