There are a couple important things to know when you lay out your pattern. You will want to maximize your fabric usage (follow the recommended pattern layout below), as well as lay your pattern pieces on grain. These instructions are based on fabric folded in half, selvages pinned together. *Sewing selvages together also works too!
MAXIMIZE FABRIC USAGE
PLACE PATTERN PIECES ON GRAIN
When you lay out your pattern it is very important that you lay it on your fabric "on grain". The most helpful tool you can use to do this is a clear ruler:
Pin your selvages together, and orient the folded fabric along the 90 degree angle of a table edge. This is a quick way to make sure the fibers are running perpendicular to one another. If you notice there are ripples on your folded egde/this step seemed oddly difficult, your fabric may be a little warped (fibers not oriented 90 degrees to one another) and may need to be blocked. This is due to how fabric is stored on a bolt and gravity's affect on the fibers.
Roughly lay out ALL of your fabric pieces to make sure they all fit before you secure/cut pattern pieces. This will save you time and future headaches.
When you lay down your pattern pieces the most important take away from this tutorial is you want to orient the grainline indicated on the pattern piece parallel to the selvage or "lengthwise grain" of your fabric. You must do this by measuring the distance from the grainline to your selvage at multiple points (ends and middle points of line). If you eyeball this it is more than likely it will be off grain and your garment may end up hanging a bit crooked or twisting.
Pattern pieces that indicate "place on fold" must be placed on the folded edge of the fabric. This piece is convenient because your fold will be already parallel be to your selvage so this step does not really require measuring. Just make sure your fold is perfectly on the edge of your pattern piece so that you do not add or remove fabric (especially to a neckline) and change your pattern dimensions. This is particularly important for patterns with collars.
*Please note you may either fold your fabric in half by pinning selvages together, or lay fabric completely flat. This is a personal preference decision. Folding in half is more beginner friendly as it will yield complete pattern pieces using the fold of the fabric and create mirror images of pieces that need two pieces (for instance a left and right sleeve, or left and right bodice). Mirror images are most important when your fabric has a face or print only on one side.