Remove the form from the paper and stick the little circle on the back for strength. Roll the form so it will have a nice shape rather than just bulky and unmanageable. Place the form under the nail. Make sure the form fits really well underneath the nail, or else the gel will run underneath, creating a messy fix.
Apply a layer of builder gel on the entire nail. Pick up some extra gel and place it where the corner is cracked off. Make sure the gel overlaps the natural nail too, or else it will crack again. It shouldn't be too thin. Let it cure for 2 minutes. Now your cracked nail is fixed and you can continue with any design.
This will also work if your nail is split or if a larger piece has broken off.