First, make sure the hazelnuts are fresh. They should have smooth, glossy shells with no cracks or holes. Shake each nut before you shell it. Those that rattle are probably dry and stale, so throw them out.
When you have them sorted, use a nutcracker, and squeeze the hazelnut with firm, steady pressure until the shell cracks and begins to separate. You may need to turn the nut and crack it a little more to better separate the shell. Be careful to not apply too much force or you can crush the nut inside. Once the shell is cracked well, use your fingers to pry it apart.
An alternative method for cracking hazelnuts is to put them into a sealed zippered plastic bag and hit them gently with a hammer until the shells are cracked enough for you to remove with your fingers. You can also wrap the nuts in a paper towel and use a rolling pin to hit the nuts until they are cracked enough to pry open.Learn more about Nuts & Grains