Gristing is cracking/crushing the grain to expose its center (the starch). You can use
rolling pins (tip: put grains in a zip-lock bag - no mess), coffee grinders,
mills, etc. Again, home-brewers get quite detailed (fanatical ?) over
exactly how this should be done.
Just get it broken into 3-5 pieces, but don't turn it to dust or flour.
If you're using a mash tun etc, you want the gristing to pretty much keep the outer
hull of the grain intact, so that it can form a filter bed during sparging. Generally
mills are used to do this - they sorta "pop" the grain out of its skin, without
breaking stuff up too much.