Serging Long Seams in Knits

I don't often pin baste any kind of seam before stitching except the loooooong ones.  Like in this knit maxi dress.  This has armhole princess seams in both front and back--making six long seams that have to be sewn just right.

The first thing I do is lay the first two pieces, right sides together along the seam line on my cutting table.  I like to start with sewing center front to one of the side fronts.  If I cut my pattern pieces correctly, they should match up between the lowest notch at the bust and the hemline. 

If they don't, I have to pull out the paper pattern and find out what went wrong.  Sometimes the paper pattern shifted during cutting and I didn't notice, sometimes there are errors in the pattern draft itself, and sometimes I have no idea what happened.
 
In a dress like this, it's usually possible to trim away the excess from the hem, as long as the bust notch, the waist curve and the hip curve match up properly.  If they don't, you'll have to recut the pattern piece. That's why it's such a good idea to check the pattern pieces before you start.

You should not  stretch your dress pieces on a long seam like this to get them to match.  Ever. Stretching either one will make your long seams ripply and, to be honest, look pretty ugly.

Once you have the pattern pieces lined up at the seams, place one pin at the under bust notch and one pin at the hemline.  Find an approximate halfway point--move up from the skirt hem, keeping the two pattern piece seam lines lined up and lying smoothly--and place one pin there.  This is the minimum number of pins for a long seam.  I like to use two more--one at each halfway point between the middle and end pins.

Next, I line up the notches above the bust and pin.  Since these two curves go in different directions for bust shaping, your seam line won't match up here and you may have to do a bit of stretching in this area--that's okay since the fabric is going to stretch over your bust there anyway.

It seems like it might save time to pin all the pieces together and then sit at the machine and sew.  You can try this if you like, but I've always found that I get stabbed by the pins in the seams I'm not working on and everything kind of gets all tangled up.  I prefer to pin one seam and then sew it.

Starting from the armhole, slowly sew to before the first notch.  DO NOT STITCH OVER THE PIN!!!  You can do some major damage to the knives of your serger, not to mention knocking out the timing or breaking a looper if you run pins through your machine.  Remove the pin and continue stitching.  You may have to stretch to get the fabric to lie smooth at these opposing curves.  Make sure the edges of the fabric line up with one another--sew slowly and take your time.  Once you've trimmed the seam allowance, you can't put it back, so make sure you only trim off what's allowed.  Remove the pin at the lower bust notch.

Hold the fabric at the next pin in your left hand.  Pull it toward you so that it is smooth but not taut.  Use your right hand to line up the fabric edges and to guide the fabric through the machine.  Do this in stages--at each pin, reposition your hands and use both of them to guide the fabric through the machine.  DON'T STRETCH!

Repeat this with the remaining seams and start all the seams from the top of the garment.  Directional sewing (sewing all seams going in the same direction) really does matter.

And after the seams are all sewn, don't forget to press.