When I was teaching machine applique'...I alway made sure the students used "TOTALLY STABLE" on the back side of their applique' projects. It's an iron on "slicky is sticky" stabilizer that can be torn off after the applique' is finished. I use it all the time and it works great. Remember when turing corners to needle down, lift foot, turn as needed, foot down and stitch. Inside corners, needle down on the inside, lift foot and turn...outside corners, needle down on outside, lift foot and turn. On curves and circles, slow and easy, inside curves, needle outside, turn slightly and often....inside curve, needle inside, turn slightly and often. Doing points is tricky( like with a star or triangle), outside point, go past one or two stitches, needle down ,turn 1/2 way, stitch one or two stitches, needle down, turn rest of the way and stitch....do just the same, but on the inside with an inside turn in the star or triangle. I'm sure there are some very good tutes on line that you might watch as well.
Good Luck