Don't let the size of it scare you away, it's really pretty easy. I built this over about three weekends in my spare time.
Following these directions you should be able to build a freestanding greenhouse also.
Of course the first thing you need to do is design your greenhouse so that you know what materials to buy. I built this greenhouse similar to the previous two projects. Build a frame structure and set glass/plastic panels in-between the studs.
Here are my drawings for this greenhouse. Design your frame similar to what I did here. Keep in mind that you will be inserting glass/plastic panels in-between your studs. For the roof I just mounted the glass/plastic panels to the tops of the studs.
I realize this is hard to see but it's a little easier on my web page. This green house is 8' wide and 4' deep.
To figure out your material needs measure for all your 2" X 4"s. All of the basic structure is made from 2" X 4"s. The door is made from 2" X 4"s without any 2" X 2"s. I made my door a Dutch door to help with ventilation. You will need 2" X 2"s to frame each window on the sides and roof. I nailed that studs together and screwed the window frames together and into the studs.
You will need a foundation which can be concrete or wood. I prefer wood because it's so easy to install. You don't need to mess with forms, rebar, concrete mixing, etc. For this project we'll use a wood foundation using 6" X 6" landscape timbers. I imagine you could use railroad ties or any other well treated wood although I don't like the smell of creosote. If you cut the wood remember to treat the ends with a preservative.
Depending on your soil different approaches should be used. Dig a trench 3" - 6" bigger than your timber on all three sides. If you have loose or soft soil dig the trench a little deeper so you can pack some crushed rocks underneath. Put in a layer of small or crushed rock in the bottom and pack it good. Get it good and level and place the timber on top. I like to have the timber an inch or so above ground level. Put rock on both sides of the timber and pack it well. This should make a good foundation for your project.