Before you place your wedge onto fabric, be sure your fabric is pressed. To pre-shrink or not pre-shrink...that's an ongoing debate among quilters. Side stepping the issue, I suggest you at least press your fabric, even with a bit of steam, to be sure your wedges won't shrink unevenly once you've sewn them together. Personally I like to spray starch the fabric to give it a crisp feel. It's so much easier to handle bias cut wedges if they've been stabilized with a little spray starch or sizing.
Consistency when cutting wedges is important. Use the inch lines on the tool to help you. If you have problems cutting wedges on an angle, try marking a second angled parallel line directly on the wedge. This can be very helpful.