We have a Harbor Freight 6 x 8 greenhouse. We like to use it to extend some summer tomatoes and   peppers, eventually it gets too cold and we just grown salad items.  Early February we start the vegetable and flowers to be ready to plant out in the garden after the last front. To winterize, first clean out everything, look for any insects, wash down the walls. Then I order a roll of bubble wrap from Amazon, ( actually one roll can last a couple of years ) you want the stuff with the largest \”bubbles\” possible. I cut into pieces matching the greenhouse glazing ( glass or panels) I have found a really nice spray glue at truvalue that is temporary and it works prefect for the bubble wrap. I spray each piece and they apply to the glazing..respraying as necessary, bubble side to the glazing. Bubble wrap on the ceiling Back wall…sorry picture seems dark and the tomatoes seem ghostly! Sometimes an edge comes unglued and I just respray it, this mostly happens on last years bubble wrap…oh well!  I put extra pieces of insulation or bubble wrap around the plants, under the shelves. I have found that the floor gets pretty cold, even freezing at night. On the North side of the greenhouse, I use foam board with the shiny foil side back towards the inside of the greenhouse Lastly in the doorway, we put a long \”curtain\” of bubblewrap in front of the door. Sometimes in the winter it can be 10 degrees and toasty warm in the greenhouse… this just buffers the doorway a bit, when someone is passing in and out! You can see this on the left side of the above picture! We have a small radiator heater on a time for nighttime only and very cold non-sunny days. But it really warms up with just the least mount of sun!  ‘Greenhouse Ready for Winter’