Depends on the extent of the crack but for an average one where the d'cell is not completely destroyed and the styro under is not too soft:

Sand out the crack to remove all the glass and just a smidge (like 0.5mm) of the d'cell. Best way for a first time effort is to use about 40grade paper rolled around your index finger - power tools may go too far too quick and take some experience.
Take your sanding well past the end of the crack
For say a 100mm long crack you will now have an area about 150mm x 15mm bare (down to dinivycell)..... and you won't be able to see any crack anymore

Then bevel the edges so you have a nice long gradual blending between deck and d'cell. Now we have an area about 170mm x 50mm.
You should be able to see "lines" of paint, then glass, then glass again then d'cell. Like if you cut some 5-ply on an angle. If you can't see all the layers you have not bevelled enough.
A dremel or similar with a sanding drum is awesome to get a nice even bevel.
Now you have a wide vee in the deck that is only 3-4mm deep in the middle and 50mm wide - a very shallow vee

Glass with a couple of layers that are the size of the patch of exposed d'cell (that is, a lot smaller than the size of the whole area you've sanded out), then a couple of layers that are bigger - a bit larger than the hole - but you only wet out the bigger patches to the edge of the hole (where it meets the paint).

Then when you cut the excess glass off your glass job is so damn close to the deck of the board it should only need a rough sand with about 100 grade and a little bit of filling with epoxy and q-cell (or epoxy and talc if you are a cheapskate and can stand the baby powder ****y smell)

A lot of people just cover the nose with plastic, garbage bag or whatever, then lay up really thick glass (about 10 layers of your 4oz) all over. Then once cured, remove and cut into a nice big U shape, sand and fill so it looks pretty (which takes forever) and affix with silicone so it is sorta removable.

Hey whoever gave me a red thumb -

wanna expand upon that? Maybe tell us all where I went oh-so-wrong