Fortunately, it's not difficult to make bias tape out of the fabric you're working with. In order to do this, you will need to buy extra fabric -- bias tape cannot be made out of the "scraps" that are left over when you're done cutting out a pattern. Since you want to cut out the fabric at a 45° angle, you want to get as much fabric extra as the fabric is wide. e.g: If your fabric is 60" wide, you will need a full 1⅞ extra yards of fabric to make the bias tape (1½ yds for 45" fabric). Plus, you need to get a few inches extra for shrinking as you will be washing and drying the fabric. Depending on how expensive your fabric is, that could be a serious investment! For your first few rounds of making bias tape, I heartily recommend using clearance/discount fabric.
The problem with rotary cutters is you can only cut in one direction ... "back and forth" will result in the blade coming away from your guide and generally shredding your fabric and making the bias tape unusable. With some practice, you can master getting enough pressure to cut through both layers without coming away from your guide, but you'll want to practice on some expendable strips before you use one of these for "production."
The good news is first that your regular sewing shears will do the job just as well, and if you invest in a rotary cutter, that even with 45" wide fabric, you will have plenty of bias tape to finish just about any project after cutting out just 3-4 of your strips, so as long as you get a few of those to come out right, you can use the rest of your fabric to practice for the next time.
Step 1: Fold your fabric into a right triangle
After you've washed and ironed your fabric (you don't have to fold it in half when you iron like you normally would), lay it out on your work surface. Take the upper corner of one edge and fold it down to the other corner. If the fabric is a little longer than it is wide, fold it so that the raw edge of the corner is laying flush with the selvage on the other side. This will create a right triangle. If you have any fabric left over, you can trim this away. (If your fabric is wider than it is long -- you can still do this, but you'll want to make sure that you still have the fold at a 45° angle).
Step 2: Mark 1" lengths parallel to your hypotenuse, cut along lines
Once you've got your triangle, you can start marking 1" lengths. From the folded edge (the hypotenuse), start by marking a ½" line parallel to the fold, (since it's on the fold, it will open up to a full inch), and then from there, mark 1" lengths all the way down.
Cut along these lines, creating long strips of fabric cut on a bias.
With the right sides together, create a plain seam to join the strips into one long strip. You don't need to bind the edges of the strips, they'll be enclosed when you stitch them into the final product.
If you're using a rotary cutter for the first time, start at the "bottom" (the shortest strips) and practice on those first, cut off 2-3 and see how you do with them before you continue.
Step 4: Iron the gatefold
Lay the long strip wrong-side-up on your ironing board, use your seam gauge to turn ¼" (5mm) of the long edge over all the way along the strip. Once you've done this edge, turn the other side in until it touches (but doesn't overlap). You can eyeball this to "meet in the middle" to the other side you just ironed, but keep your seam gauge handy, and make sure that fold-to-fold is ½" wide (10mm).
Done! Your single-fold bias tape is now ready to use!
Finishing a raw edge with single-fold bias tape
Let's make a simple sleeveless top (Simplicity 2262-D)
Let's make a sleeveless sundress (Simplicity 2938-A)