I am cardboard building freak. Really depends on what you plan to do with the coffin.
If you want it to function like a real coffin, its going to take a lot of hot glue and bracing pieces. If closed and non functional then not as much. I have built two of my sons transformer costumes from cardboard and one year I built tons of monuments with cardboard. As far as I know there isnt a tutorial. I will post some pics of some of the above mentioned items.
Hubby made one a few years ago. He ripped some scrap wood down into thinner pieces, nailed it into a toe pincher frame shape, stapled on cardboard to outside of frame. Spray painted exterior cardboard,stapled an old quilt to the insides to hide frame: done. Lid was hinged: nailed on one side, hot glued to the cardboard on the other. The frame was very rough and roughly measured... nothing fancy. He just sketched out dimensions on a paper towel & hit up the table saw. It worked great, but we got rid of it after 3 years because storing it was a PITA. It wasn't sturdy enough to store anything ON it or IN it, so we had to treat it gently. Forget that...we now have a wooden one that is heavy, but it pays for itself by being storable without headaches.
Gotta agree with Debbie, wooden coffins are the best. I have built those from plywood sheets and from 1x12 pine boards. I prefer the pine board ones and they are great for storing stuff in. As for where to put them between seasons, we use them as coffee tables.
The cardboard ones work fine as long as weather and wind aren't a major issue. As Roxy stated, if they are more for show than active use, then they will do fine.
For storing wood coffins and toe pinchers, if you build them using drop pin hinges rather than as rigid pieces, you can pull the pins so that they can be stored laying flat. If you wish to use cloth or quilted liners, you can use velcro to hold the pieces in place. This lets you wash and store the pieces safely and easily.