Courtesy of SimplyCanning.com
First harvest your spinach. The best time to do this is early in the morning when it is at its freshest. Spinach will be crispier and fresher before the heat of the day hits it.
You can harvest spinach by cutting or pinching off the larger leaves, or cut the whole plant 2 inches or so above the ground. The plant will grow new leaves and you can harvest again in a few weeks.
Rinse the spinach well. Sorting out any weeds, browned or damaged leaves or other things (bugs) that might have gotten mixed in. Spinach will often have dirt hiding in its leaves so you need to rinse well. I'll rinse and drain several times before I am content that all dirt, bugs or dead leaves are taken care of.
Spinach and other greens need to be blanched before freezing. This helps retain the color and nutrition. Bring enough water to cover your spinach to a boil. I use my blancher but a large pot will work as well. When the water is boiling throw in the spinach use a spoon to lightly push the spinach down so it is all down in the water. Place the lid on to hold in any steam. Start timing right away for 2 minutes.
Read more here.