Getting a proper crack is important for getting the best mash efficiency and extraction rate. If cracked grain is too finely ground it will turn into porridge. If the grains aren’t cracked enough it will result in a poor conversion rate.
A properly cracked grain has a range of particle sizes that create a compromise between extraction and lautering. You are looking to crack just the outer shell of the grain. A mill is the ideal way to crack grain, but a rolling pin, beer bottle, etc., will work as well. Just place the grains in a zip lock bag and crack away. You do not want to crush the grain so much that flour is made.