As CKcowl said, with rulers. I have done it with a ruler as small as 6" x 2". One word of advice... Easiest and fastest does not always look best. Depending on how large the area is you want to cross hatch you may wish to premark. If the area is bigger, like bigger than your exposed area for quilting without advancing the quilt, it is very easy for your crosshatch lines to stray off the straight line and ever so gradually slant off. Especially when using a smaller size ruler. If you are just doing a small area, like the background of a pieced block or backfilling a motif, than you can easily do it without premarking as long as you have some point of reference for lining up your ruler, like a seam line or something.
You will also need someplace to travel because it is crazy to stop and tie off every line. I usually travel in the ditch, along a perpendicular line or back track along a motif but be careful of thread buildup. It will show quite a bit if you are using a heavier weight thread and will most definitely show if you are using a varigated. Look at this topic I posted a while back, you can see how I premarked the quilt and travelled in a perpendicular line. The rest of my travelling I did around the applique where it sunk in and wasn't visible or along the already quilted leaves and vines. I also used a very thin thread so the travelling build up wasn't obvious. This I did with So Fine by Superior.
http://www.quiltingboard.com/picture...k-t210274.html