So you made sure your fabric is cut on grain, now it is time to make sure the fabric is oriented "on grain".
When your fabric is oriented on grain they fibers will be running perpendicular to each other at a 90 degree angle. When you need to correct a warped textile oriented at a slight angle as picture above, it is called “Blocking”. This is important to make sure the fibers are running in the proper direction on your finished garment. If your horizontal grain is not oriented at a 90 degree angle to your lengthwise grain, your garment will most likely fall in a crooked manner or twist on your body.
A Quick Illustration to show how to block your fabric. Note the ripples along the fold of the fabric that is "off grain". That will be a reliable indication you need to block your fabric.
Fabric scrap pictured below is "on grain". Warp and weft are oriented at 90 degree angles.
When folded in half in preparation for pattern, it folds nicely (as pictured blow).
Here is a depiction of fabric that is "off grain." Note the orientation, the warp and weft is not at 90 degrees. This may happen to fabric due to the nature of its storage wrapped on a bolt folded in half, or gravity if stored on a bolt vertically.
This is an extreme circumstance but note the outcome of this scrap when folded in half:
To combat this problem, you may pull along bias of fabric in manner shown below to orient fabric back "on grain". You may need a helping hand for larger fabric sections.
Once Your fabric is blocked, you are ready to place pattern pieces on the fabric. An ideal method when working on a home table is to fold fabric in half and pin selvage together. Always pin ends first, then middle, then pin in between. Use the 90 degree angle on the edge of a table to correctly orient your fabric.
Make sure all edges are lined up:
At this point you are ready to lay your pattern pieces on your fabric!