Cutting pattern pieces on the grain line for woven fabrics is the most important step in the sewing process.

New sewers may not know what that means or why it’s so important.  Quite simply, if your pattern pieces are not cut on the grain line, the finished garment will twist and drape sloppily.  

The main reason fabric is off grain is how it’s rolled onto a bolt.  As the roll gets bigger the upper layer of fabric will tend to shift out of line.  Fabrics of a lesser quality also tend to be off grain.

There are two grain lines on your fabric. The weft (the cross grain) and the warp (the length grain). The warp grain is parallel to your selvages and the weft runs across perpendicular to the selvages. The bias on your fabric runs at a 45 degree angle to these grainlines. The warp thread is generally stronger and is least likely to stretch out of shape.

The warp grain or selvage side does not need to be straightened – only the weft or cross grain.

From my experience, the best method to straighten the cross grain is by pulling a thread.

Here’s how.

Make a snip at one selvage edge and pull out one thread. Start gently pulling the thread out all the way across the fabric. 
If you’re lucky it will come out in one go but it might bunch up a little while you are pulling it so just pull every few inches and then cut on the line it has created until you reach the end.
When you pull out the single thread it will give you a nice straight line across the fabric. Using this line as a guide you can cut all the way across the fabric.
Once that step is completed, I pull one more thread to ensure it comes all across in one strand.  Even cutting on the pulled line sometimes goes off a thread or two.
Now not only is your fabric on the straight grain, it is ready for the next step of pre-washing/pre-shrinking to prepare your fabric for cutting.  If the grain line is straightened prior to washing, there will not be crazy, random fraying from the washing cycle.

Now your fabric is ready for the next most important step.  

Step 2:  Pre-Wash | Pre-Shrink Your Fabric