Find the cutting layout diagram which is appropriate for the width of fabric that you purchased. The first page has the diagram if your fabric is 45" wide. To use this layout you will need to lay the fabric out smoothly with it not folded. Pay close attention to turning the pattern pieces over as it was explained in the directions.
The second page has the layout diagram for 58" to 60" wide fabric. The wording below the diagram tells you what size for which lay out. It helps to circle the diagram you will be working from, to prevent confusion. These layouts use the fabric folded as it was probably found in the store. Pay attention to the folded edge verses the salvage edge in the diagram.

Follow the diagram to roughly lay out the pieces. Now you need your yard stick or straight measuring devise. Line up the end of the yardstick with the fabric salvage edge. Measure to one end of the arrow on one end of the pattern piece. Now measure the same way for the other end of the arrow. These measurements MUST be identical.
Measure again to check you accuracy and make sure all the pieces are flat and smooth. Now pin the edges of the pattern pieces, smoothing the pattern pieces as you pin. Measure again and make sure all the pieces are flat and smooth before you pin the entire piece.

Pinning seems like, "Who could possibly do it wrong?". The answer.. plenty of people! It is necessary to keep the pattern and the fabric smooth and flat while you pin. All of the layers need to be in unison. Granted, fitting may not be critical in a bathrobe, but it is best to get in the habit of working correctly. Read more about accuracy and why it is important at Design, Accuracy and Alterations For Sewing Your Own Clothing.

Some of the pattern pieces are designed to be placed on a fold. This picture shows how to line up the pattern piece and the edge of the pattern piece. This fold is not cut. For the 45" wide fabric, the pieces are turned over and the fold line is not cut.
Now is the time to even double and triple check your pinning and measuring. It is much safer to double and triple check before you pick up your scissors. Once you have cut and error may not be able to be re-done without purchasing more fabric.

Next lesson: Marking