3 cattle panels ($22.00 each)  You will use 2 and 1/2   ($66.00)
about 15 - 8 ft. 2 by 4s (I paid around $2.50 for each)        ($37.00)
1 - 8 ft long 2X2
Liquid Nails and flat L-Brackets

You can buy a roll of plastic at the hardware store but it won't be crystal clear. I wanted my greenhouse to look like glass so I used the heavy plastic from Walmart that you buy in the fabric department.  There are several thicknesses available but I found the one that sells for $2.99 a yard is just right. It is as clear as glass and comes in a width of 53 inches which is just about perfect for this greenhouse.

6 yards for roof  (2.99 a yard )                                   ($18.00)
9 yards for sides                                                       ($27.00)

The greenhouse is 8 ft by 6 ft. The roof is at least 7 feet tall in the center so you could easily make this 8ft by 7 ft and still have plenty of head room.  The sides are 50 inches tall, (the width of the cattle panel)

Cattle panels are 16 ft long by 50 inches wide so the first thing you do is cut 2 of them in half.  (8 ft by 50 in.) This gives you the long sides and the roof.  The back wall is 6 ft wide so cut a piece from the third cattle panel  (6 ft by 50 in.) You'll have a 10 ft piece of cattle panel left over to use for some other cool project.

Make 2 frames:  8ft by 6 ft  ( Use 2- 8ft boards and cut 2 at 69 inches for the 6 ft side)

Cut 4 boards at 43 inches for the upright boards on the corners.  (Allowing for the 2 by 4s)
(43 plus 3 1/2 plus 3 1/2=50 in.)

Nail or screw the frame together.( Make 2)
Attach the upright boards. ( I used Liquid nails and L-Brackets but you could toe-nail them)
Put the top frame on. (Liqid nails and L-Brackets)
Attach 8 ft cattle panels to sides and 6 ft cattle panel to back. ( I used those big staples) nail the cattle panels to the 2 by 4 frame.This will "square up" the greenhouse for you. 

Use strong zip ties to connect the two cattle panels together for the roof.  Make sure they are perfectly matched together-I secured them in the middle with wire to make sure they didn't "shift" to one side or the other...which is what they try to do.  Don't tighten the zip ties too much until you get the roof on.

Put the roof on and attach to the sides with zip ties. (again, make sure they are squared up- don't let them shift)

The gabled ends are cut at 50 inches with a miter of 45 degrees on each end. Cut 1and1/2 inches from the top of each board so you have a place to rest the 2X2 on top of the gable. (See picture) Slide the gable boards in to the ends of the frame and nail to the cattle panel roof. These are not really supporting anything, they are mostly just so you have a wood frame to staple the plastic on.  Use a 2X2 8 ft long for the top of the gable.

The easiest way to cut the frame for the door (and the rear window) is by holding a 2X4 up to the greenhouse and marking the angle and height. Same for the top of the door frame.  My door is set in the frame but it's better to have the door frame nailed or screwed on the outside of the frame.

You can make the door out of anything you have around.  It's basically a frame that fits in the opening you create.  I used an old storm window and a headboard from a youth bed. Same with the window- make it any size you want. Get creative!

Okay, this is the tricky part...you'll need to cut out the piece of the front top frame- for the door.  I used a reciprocating saw and it wasn't very accurate.  Maybe you can come up with a better way to do that part. 

I secured the door frame with L-Brackets and Liquid Nails. (Since the door frame is supporting the weight of the door, I used L-Brackets and corner braces.

Okay, now you're ready to put the plastic cover on.  Start at one side of the door and line up the plastic with the top of the side frame. Staple it on all the way around to the other side of the door.  (Keep it tight and straight along the 2X4 framing)  Cut the excess off.  The roof is next and it will overlap the top and sides by about an inch and a half. Do one side of the roof first. Roll out the plastic on one side of the roof and line it up  with about 1 and 1/2 inches overhanging the top and the side. Staple the plastic to the frame at about 6 inch intervals.  Do the same on the other side of the roof. (Make sure you have the roof plastic on top of the side plastic - for waterproofing)  Cut smaller pieces to fit the area above the door and around the window in the back, making sure it is overlapped by the roof plastic and that it overlaps the side plastic.

Tips and things to remember:
Make sure the "sharp" ends of the zip ties are facing away from where the plastic will be.
I tipped the whole greenhouse on it's side so I could apply and staple the plastic on the roof.
Paint the whole frame before applying the plastic.
Use bolt cutters to cut the cattle panels.  Available at "Harbor Freight",  pretty cheap.
Put hinges on the door and on the window.  
The window should be hinged on the top so it opens from the bottom.  (It will keep the rain out)
Don't cut the 2X4 out at the bottom of the doorway until your greenhouse is in it's permanent spot.
Put a handle and hook and eye closure on the door and you're done!  A great looking greenhouse for about 150.00