I agree with Mariposa.....shoulders down.....deep breath and just jump in!
All I can tell you is to "do your best" and it will be fine. After you get a few quilts under your belt, you will notice that "your best" just keeps looking better and better. The mistakes are little surprises that prove that the quilt is made by a human being (not an automated robot). If you do your best, are proud of your work and ENJOY yourself.......you will be fine. Go for it.....you can do it!