Before I use a commercial pattern, I true it. By commercial pattern, I mean a Simplicity, Vogue, McCall's, Butterick, KwikSew, or any other pattern normally bought in a retail fabric store.

Why do I true the pattern?I true the pattern to make sure that seams will sew together correctly, and to bring the pattern closer to industry specifications.

First a few differences between commercial patterns and industry patterns. A pattern intended for home use will usually have 5/8" seam allowances all around. In the apparel industry, seam allowances vary. There is no standard; rather, some areas of a garment may have ¼" seam allowances, while others have ⅜" or ½". There are various reasons behind these different widths. Mostly they have to do with sewing the garment quickly. In a factory, workers cannot take the time to grade or trim seam allowances. There are other subtle differences. These include the amount of ease built into sleeve caps, facings, pockets, and collars.

I used to wonder why the clothes I sewed from commercial patterns never looked quite right or like a retail garment, and I think that the lack of truing on commercial patterns is a lot of it.  Using the tips below, your garments made from trued patterns should look nicer and more like retail finishing once completed.

Tools you will need:  Fine point Felt Tip Markers in Red, Blue, and Green; Paper; Scotch Tape; 18" See-Thru Ruler (like a quilter's ruler); French Curve or a Hip Curve.

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First, open up the pattern and find all the pieces that you need to construct your garment.  Cut them apart from each other.  Leave the excess paper or tissue around them -- you might need it as you progress.
I usually start at the shoulders.  Lay the back shoulder seam over the front shoulder seam so that the seamlines match up.  The easiest way to do this is to draw a line 1 ¼" down from the front shoulder's cutting line.  Use a green marker to do this so you can easily tell your line from the pattern's.  Then lay the back shoulder seam's cutting line along your newly drawn line.  Check the inside curve at the neck and the curve at the top of the armscye.  They should be nice smooth curves, with no "peaks" at the seamline.  If they come to a peak (and most of the time they do), take a French curve and with a red marker draw a nice smooth curve, blending in to the old cut lines.  Along the neckline you can also trim the seam allowance down to ¼", if you want.  This makes it easier to sew the neck seams (just remember to do it on the collar and facings, too!).  I do this with red marker too -- using a see-thru ruler set in ⅜" from the cutting edge I follow the cutting line and draw a new one in red marker.  If you've drawn a new curve -- remember to follow it, not the old one.
Repeat this process at the underarm seam.  The underarm seam at the armscye may form a nice egg shape.  If the sleeve is a drop-sleeve style, it will come to a severe point, and that's okay -- just leave it.  The more tight fitting the garment, the more important it is to have that nice egg shape.  If you have an empire waist, or a regular waist seam, be sure to check for a peak there.  The waist should be a smooth, slightly curved line that flows from the front to the back.  If it doesn't, then use your curve to draw it in using the red marker.
Draw in the seam intersections at the underarm and shoulder seam of the front and back armscye.  I use the see-thru ruler here -- set the ruler in ⅝" from the cutting lines and draw intersections (crosses) -- use blue marker here.  Then, also using blue and the see-thru ruler, tick off the sewing line around the armscye curves.  Measure the seam lines of both the front and back -- write these measurements down.  The front and back should be the same or within ½" of each other.  If one is much longer than the other, you need to adjust the front and back to get them to within ½" of each other.  Just be sure that after you have made adjustments you still have the nice curves at the top and bottom
Using blue ink, tick off the seamline of the sleeve cap.  Measure it and compare it to the front and back armscye measurements.  There is no need to do this if the sleeve is gathered at the cap.  For eased caps, though, you should have no more than 1 ½" difference between the sleeve cap and armscye measurements.  Adjust the cap if there is too much ease (If you don't need the fabric in the sleeve, you can fold it out and then redraw the curve at the cap).  Too much ease means puckered sleeve caps and a difficult time sewing the seam.  It also makes the sleeve and therefore the whole garment look "home-made."   Often plus size patterns (larger than a 16) will have far too much ease at the sleeve cap.  I also check to make sure that the collar and the neck opening measure the same (you can walk them off or measure their seamlines).  Also check to be sure that the skirt and the waist seam of the bodice have the same seamline measurements (remember to account for darts, tucks, etc.)
On Shirts and blouses, I often make up a collar sample and check to see how the collar lays.  If necessary, add more distance at the outer edge of the collar by splitting the collar to the neck seam and adding a bit of paper.  Usually a few wedges 1/8" each will be more than enough to get the collar to lay correctly.

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This list by no means covers every aspect of every pattern. It's meant to give you a starting point. After you've checked a few patterns, you'll see where the problem places are, time and again. These little things add up to a nicer looking garment in the long run.

I hope that my drawings help illustrate what I've written. I'm no artist (maybe that's why I'm not a famous designer?) but they should help demonstrate the process. E-mail me if you have questions.

Now go sew something!