yes, and it's not something I do well or often. It was done frequently in sewing garments and I learned there I didn't want to do it if I didn't have to. you have to carefully mark on the L shaped fabric where the two seamlines meet at the corner, and also mark where the seamlines meet on the square corner block - the part that fits into the L. match these two points up and start your seam. Turning at the corner can be awkward, so many seamstresses start seams at the inside corner by putting the needle down exactly where the two points meet (the ones you marked carefully) and then stitching toward the outside of the seam. Do first one side then the other, both times starting in the center and working out so you make sure that the stitch gets right into that point.
If you want to press the seam allowance toward the L shaped fabric, you'll need to clip INTO the seam allowance at the corner of the L. If you want to press the seam allowance toward the square, you'll probably need to cut the corner of the square diagonally to get rid of bulk or clip into the seam allowance on each side of the corner point so that the excess fabric can lap over the excess fabric next to it. Does that make sense?