If you are doing an outside corner, you need to sew to where the needles come to the end of the fabric, then lift the presser foot and turn carefully and continue sewing or you can just sew off and start over and use fray check or fray block to seal the threads at the corner. For an inside round corner you need to try to ease around the corner by straightening the fabric as much as possible. This all takes a bit of practice and experimenting, but you will get the concept if you try. I would also suggest taking a serger class, if you have one offered in your area. You will get more ideas than you can imagine in just a several hour class.
Saskatoon, Saskatchewan Canada Janome 15000, 12000, 4120, 760, 1200D Serger, Huskylock 926, Cover Pro 1000 & a few older sewing machines; my favourite computer games of DigitizerMBX v.4.5 with Cutwork, Floriani Total Control, & EQ7