Sewing the opening closed where you turned a bag right side out isn't difficult, but it can certainly be ugly and obvious if not done well! Here is an easy way to get nice results when slip stitching your seams closed.
1. Sew the opening closed first. Whenever pattern instructions say "leave an opening for turning" go ahead and sew it closed first. If you follow the instructions below you'll have a nice crease to follow when it is time to sew it shut.
Back stitch where your opening will begin. Then lengthen your stitch (makes it easier to remove the stitches later) and sew the length of your opening. At the end, back stitch again.
Now warm up your iron and press this seam open flat.
Using your seam ripper or a small pair of scissors, remove the long stitches between the back stitching on both ends.
Viola! You now have a nice crease to follow when slip stitching your opening closed. Even if it isn't perfectly pressed you should still be able to see the faint stitch line left from the removed stitches, which you can then use as your guide.
2. Close the seam.
Thread your needle and tie a knot in the end. Insert your needle from the underside of the crease coming out right in the crease line. I start in the middle of the opening. Pull the thread through so the knot is resting on the back side of the crease out of sight.
Directly on the other side of your seam, insert the needle and gently but firmly pull the fabric together. Slide the needle through the underside of the casing created by the crease and come up out of the fabric about 1/4" from the insertion point.
Insert the needle again directly across from where you just came up out of the fabric. Slip the needle through the casing again another 1/4" or so and come up. Repeat this the length of the opening.
If you started in the middle you will need to turn at the end and go back to close the other side. I usually go back and forth across the opening area a few times. The first time I make just a few stitches that are widely spaced apart. Then when I turn around and come back I sew in between the existing stitches. It seems easier to get a nice even look doing it this way. You can use the second--or even third--set of stitches to fine tune and smooth out any parts that might get a little bumpy.
Once the opening is completely closed tie off on one of the existing stitches by coming out of the fabric right next to one of the stitches and then inserting your needle under the stitch. This will leave you with a loop of thread you can pass your needle through to create a knot.
Once the knot is in place insert your needle into the fabric right next to the knot and slip the needle through the casing about 1/4" -1/2" and then come out. Pull the thread firmly and the knot will be pulled down into the crease making it less visible. Snip your thread and you're done.