Bring a large pot of water to a boil. Alternatively, you can use a vegetable steamer basket over a couple inches of boiling water.
Cut the broccoli into florets. Wash and dry. Fill a large mixing bowl with ice water.
Once the water is boiling, drop the broccoli into the pot and cook for 3 minutes. If using a steamer basket, cook for 5 minutes. Do not overcook the broccoli. It should still be fairly firm, just gently cooked.
Immediately transfer the broccoli to the bowl of ice water. You can use a slotted spoon to do this quickly or drain the broccoli first then transfer it.
Let the broccoli sit in the water for a few minutes before draining. Get the broccoli as dry as possible. I often will squeeze the florets gently in my fists to ring out any excess water.
Spread the blanched and dry florets out on a baking sheet in a single layer. Place the baking sheet in the freezer, somewhere it won't get bumped, and freeze for 1 to 2 hours until frozen solid.
Transfer the frozen broccoli florets to a freezer bag. Label and store in the freezer for up to 6 months. Use within 3 months for best results. Add directly to cooked dishes without thawing first. Do not use in raw broccoli salads or else it will be mushy. I also recommend steaming instead of boiling if you are using it for a simple side dish.
To freeze broccoli without blanching, just cut it up and store in a freezer bag. Remove as much air as possible. Freeze and use within 1 month for best results.