What Seam Do I Use

Flat-felled Seams are best for clothes that will receive a lot of wear, like jeans or children’s clothes. 
1.   Sew a seam and press it open, then press it to one side.
2.  Trim the inner layer to 1/8 inch.  I like Applique  scissors for this task, they keep me from cutting the wrong layer. 
3.  Press the outer layer to ¼ inch and lay it down over the 1/8 inch layer, completely covering it.
4.  Sew the ¼ inch layer down, making sure the stitches are straight; they’ll be seen from the right side of the garment.  Use Sharps machine needles for a perfect straight top stitch.

French Seams are best for sheer fabrics.  Sometimes the overlock is too cumbersome for sheers, and sometimes you don’t want to see all those threads through the fabric.  French seams take a steady hand but look so beautiful they’re worth the effort.  Make sure you use a machine needle sized for sheer fabrics.
1.  Start out with the wrong sides of the fabric together.  This takes some thought sometimes, because it goes against our natural thinking.
2.   Sew a seam half of the width of the desired seam. 
3.  Trim both layers of the seam to 1/8 inch.  I find that a pair of sharp quality scissors are indispensable.  I like to use 6 inch or shorter scissor blades for fine work like sheers, as the larger blades tend to get unwieldly when cutting handwork. 
4.  Press the seam open, then fold the fabric so that the right sides are together and press again so that the seam is in the center of the fold.
5.  Stitch the seam (the other) half of the width of the desired seam. 

Serged Seams are done with an overlock machine, and are great for a variety of garments.  They make great seams for knits.  (Make sure you use ball point machine needles.)  They make a great finish for seams on woven fabrics because they finish off the edge and stop fraying.  As with all types of sewing, make sure you have the correct size overlock needle for your fabric. 

Use plain seams for most garments that need no special treatment. 
1.  Pin right sides together.
2.  Sew the right distance for the pattern’s required seam. 
3.  Trim the seams if their bulk gets in the way of the rest of the construction.  Again, I like smaller scissor blades for this kind of work.