Sewing first vs. serging first
There is some argument about this in the DiY community, and really, I think it depends mostly on what you're sewing, and personal preference. When in doubt, experiment, and find what works best for you.
Sew first, then serge: I think this would be a good place to start if you're a serger noob. A serger does take some adjustment when sewing. Because the machine cuts the seam allowance off as you sew, you've got a lot less room for error.
This would also be a good technique if you're sewing something like a woven fabric that doesn't need the stretchable seam that the serger allows, but you'd like a finished looking seam.
Serge first, then sew: I use this technique sometimes when I'm sewing a zipper or a slit into something, and I won't be able to have a serged seam. I serge the edges of the fabric, then sew the pieces together, so I still get a professional looking edge.
(see photo at left)
Serge first, then topstitch: This is the technique I use most often. I serge the seam, then I press it to one side and topstitch it down.
I don't topstitch all of my serged seams, but most of them. Some things are too bulky to topstitch, so I just leave it.
(topstitching pictured at right)
I've found that I need a much smaller seam allowance when I'm serging my seams. I generally use about 1/4" seam allowance when I'm serging, vs. a more standard 1/2" or 5/8" seam allowance. You will find what you're most comfortable with, just keep it in mind when you're cutting the pieces of your garment out.
What to do with the thread tail
You can't backstitch with a serger, so you might be left wondering what to do with that little thread tail.
There are several ways of dealing with it, I recommend looking at your manual and seeing what option you like best. My favorite way is to take a big fat needle (a yarn needle or very large upholstery needle will work, the less pointy the end, the better), threading the thread tail through it, and then sliding it up into the seam. (photos below)
What is the difference between overlock thread and normal sewing thread? Should I use only overlock thread with my serger?
Overlock thread is not made to be as strong as normal thread, so its cheaper to buy in those big spools. You can use normal thread in a serger, but it's a lot more expensive and unnecessary. You'll probably run out in about 20 minutes. You probably don't want to use overlock thread on a regular machine unless you're having one of those out-of-thread-at-midnight emergencies, because it isn't as strong.
How do I hem with a serger?
I personally don't mind the look of a plain old serged edge as a hem. Use a contrasting color and it kind of looks like the hem has some sort of trim at the edge.
If it's something that wouldn't look right with the serged edge, serge it (or don't), press the bottom 1/2" or so under, and just stitch around it like you normally would hem something. If you don't like the look of a straight stitch, you can use a zig-zag or another decorative stitch. Do it in a contrasting color and no one will think you were avoiding hemming it!
Some machines come with a hemming attachment, I only used mine once to test it out... I'm too lazy to switch the footplate and the foot and the settings everytime I want to hem.
Why does the serged hem of my knit garments turn out all wavy and stretched out?
You need to adjust the differential feed. On most machines it's probably numbered 0.7 to 2.0. If you serge the edge of something really stretchy with the diff. feed at 0.7, it'll make it "lettuce edged". (Like this.)
If you take some non-stretch fabric, and serge the edge with the diff. feed at 2.0, it'll actually ruffle it for you, which is kind of cool.
If you want the serged hem of something stretchy to not get all wavy like the lettuce edge, put the diff. feed a few clicks closer to the 2.0. Some knits I can do fine with on the normal setting (1.0), but if they're pretty stretchy, I'll go one or two clicks towards 2.0. If they're really really stretchy, I go even further towards 2.0.
It takes some messing around with to get it right, but once you get the hang of it, it helps a lot.
What kind of needles should I use in my machine?
Check your manual for this, as it differs from machine to machine.
HIDING THE SERGER THREAD TAIL:
Step 1. Get the biggest needle with a blunt edge that you can find. I'd plan to have a few extra around- I always seem to lose these.
Step 2. Insert the needle in the seam. Leave the eye exposed.
Step 3. Insert the thread tail into the eye of the needle. It's easier to insert all those threads if you roll it between your fingers a little first.
Step 4. Gently pull the needle through the seam.
Step 5. And out the other side.
Step 6. If there are some thread ends still poking out, just trim them with some scissors.
Step 7. Huzzah!