Determine how the 2 fabric width patterns will line up.
finger press the seam allowance back.
You can press the seam allowance, but you need to be careful not to shrink it!
On the 2nd width of fabric glue baste along a line where width #1 will butt up to the pattern match.
Be sure when using the fabric glue, use it sparingly so it does not seep through. Stitch witchery could also be used in this application. A permanent bond is not necessary.
Let the fabric glue set up so the match doesn't move when you move the fabric.
You can also set the glue basting faster by ironing it, but again, be careful not to shrink the fabric.
This crease line is where you will sew the widths together.
Stitch in the ditch of the crease line.
In this case the crease line follows the pattern on the plaid. When working with a printed pattern, the crease line is all you will have to follow.
As you can see the pin is pointing to a perfect match.
When the repeat across is even smaller on a plaid or stripe fabric, this method is even more critical to obtain a perfect match.