Work your way around the edges with plastic picks.

Take your time otherwise the glass will shatter (see second image).

If your screen is overly cracked (looks like spiderwebs) you will want to put several layers of packing tape over your screen to try to keep it in one piece and do everything in your power (go slowly and don't be shy with the heat) to prevent it from shattering more.

Caution: If your glass happens to shatter you might want to consider wearing safety glasses.

Plastic razor blades you find in a hardware store are great for intricate cleanup like this. Alternatively try guitar plectrums (picks) from a music shop. These won't scratch the digitizer.