The first rule of neat seams involves blocking each piece. I know that many people do not block the individual pieces of a garment, I find that it makes seaming much "neater". After the pieces have been blocked and are dry, start by pinning the seams together. I use wooden pins. I start by matching the ends of each seam, then add a pin every 2" or so. Weaving the outermost edge of the last/first stitch of each row to the same on the other pieces helps make for a nice seam. Take your needle and go under this outer edge (with the right sides pined together), then come up and over the edge and do the same on the next row. Try to not pull your stitches too tightly as that will cause "puckering".
You can create a selvage edge on each piece of your garment, this keeps the "rolling" under control, by knitting the first and last stitch of EVERY row. This will not affect the pattern at all, but will create a nice base for your seams. I hope I explained this clearly.
here is a good seaming video to get you started...i think this one is good but there are lots of videos with this information and everyone learns differently...try to do this version and then expand with the other suggestions offered on this page to see how to go cross-ways, on the diagonal, etc., be patient, watch a few, everyone has different levels of teaching ability, but lots of them have one or two little tricks that are great...
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