I have always had trouble making tidy corners for 3-D sewing projects. Since I worked out this technique I thought I'd share; it works well on heavier fabrics and for projects like fabric baby blocks where it really shows if you fudge the corner.
Hope this tutorial is helpful; leave a comment if you have questions.
Step 1: Cut Out Your Fabric
In this example I'm making a cube so I cut out 6 identical squares of fabric but this technique works for any project where you need three pieces to intersect in a perfect 90 degree corner.
You'll also need a sewing machine.
(I also let my daughter decorate the fabric to make blocks for her cousin--this step is optional ;)
Step 2: Sew the First 2 Pieces Together
Here's the only trick to this project; start and stop sewing 1/4 inch away from the end of the fabric. This assumes a 1/4 inch seam allowance. If you use a different seam allowance, just stop that distance from each end. For example, if your seam allowance is 1/2 inch, start and stop 1/2 inch from the ends.
Ok, so to do this, stack your pieces of fabric good sides together with the edges lined up. You can see in the photo that I drew a line to show exactly where I needed to sew (this is just for the tutorial--you don't need to do this). Plant the needle 1/4 inch in from the edge and take a few stitches. Back stitch to lock the threads and continue stitching all the way across the project.
1/4 inch from the end, plant your needle and back stitch again.
Step 3: Attach the Third Piece of Fabric
Now attach the third piece of fabric; you want them to make an "L" shape.
When you're lining up the fabric, again, put good sides together. Then line up the edges of 2 pieces and fold the seam allowance of the third piece out of the way (you can do this because you left 1/4 in at the end of the seam). Look at the photo, this is hard to describe.
When stitching this seam, once again start and stop 1/4 inch from each end of the fabric and back stitch to lock the stitches. (see the second photo)
Step 4: Sew the Last Seam
Now fold the fabrics to make the corner (this will make sense when the fabric is in your hands). Again, tuck the seam allowances out of the way and stitch. This time, on the end where all three pieces of fabric intersect, you can sew off the end. But be sure to backstitch.
Step 5: Turn Your Corner Right Side Out (plus Bonus Content)
When you finish the third seam, your project will look like the first photo (from the inside). Turn it right side out (second photo) and you're done.
If you wish to make a cube, continue adding squares to create a cube shape (it will be inside out). When you're ready to sew the last seam, stitch the corners and leave a gap along the center of the edge so you can turn the cube right side out (see third photo). You'll have to hand stitch closed the opening.