Cut off the stalk (or bottom) of the broccoli. Cut each floret into slices.
You can always add the bigger/thicker pieces on the bottom and smaller/thinner pieces at the top.
Spread the broccoli florets out on a baking sheet, so that they don’t touch each other. Flash freeze them for about an hour or two. Add them to an airtight container or resealable freezer bag with the date of when you steamed them.