If ye are lazy and just want to ho some free models then have a look at my Deviant Art site - theres a few free plans up on there.
Step 1: Materials
The program we are going to use to make the nets is called Pepakura Designer 2 - It's a japanese program and so the documentation for it is a bit squiffy. It's also shareware - you can work on a project and print it out but you can't save. There is a way around this but if you are going to be using this on a regular basis i would reccomend splashing out on the complete version. Its not much and its a way of thanking the peeps who went and made the prgram.
Pepakura Designer 2
For the initial modelling software, use what you are comfortable with - Pepakura designer accepts a wide range of filetypes (listed here). However if you as skint as me there are several freeware alternatives to the big name programs:
Metasequoia LE - This is the one i use, it's freeware and again japanese so the help files arent too hot and there are a few bugs that crop up every once in a while.
Google Sketchup *edit* I had bit of a dabble with this the other day and as people on the comments pointed out, it doesnt save natively to a format that pepakura designer can read, but you can export your models in google earth 4 format and it does work. Thanks guys.
**double edit** Aparently there's a plugin for sketchup that converts the faces into SVG format for use in inkscape http://code.google.com/p/sketchup-svg-outline-plugin/
As well as the software ye are going to need a printer and some fair meaty paper or card- theres a lot of play in the weights but go for something that will be sturdy but isnt so thick that the peices dont fit together properly. My mate allofusarelost even uses photo paper for his. Play around
You are also going to need something sharp - scalpel/ craft knife (or an "xacto" to be all brand namey). I'd go the whole hog and use a metal ruler and a cutting mat too (scrap thick cardboard also works but it needs to be big). If these scare the bejeebies out of ye or your 'rents wont let you, you could always fall back on the age old standby, scissors.
And then another dilemma... Adhesives! PVA (white glue/wood glue) works as does balsa cement (my fav when i have it) and are usefull for fiddly bits - tho pva has a tendancy of warping sometimes. You can also use a glue gun but as with those things it will always be a bit messy and "imprecise". Another option is to use double sided tape. This has excellent hold on large areas as long as you can get around the back of it but is a little time consuming cutting strips to length. When it is all prepared it is very quick to make and doesnt require you to hold pieces together for a long time whilst the glue sets.
Superglue is a nono! - it was designed for sticking together peoples not papers and you mind wind up with a cool yet useless bodymod.
Step 2: Spark Up
I'm going to use this little hairy fella that keeps popping up in my doodles - kinda a cross between doramon and a yeti O_o I'd previous turned him into a stencil for *cough* tshirt making purposes :p
Step 3: Get Your Vectors On.
Also, make sure that all your shapes are closed and connected together or you'll get angry red lines in pepakura that means that you wont be able to join the two edges together.
Step 4: Unfold
Click the magic button "unfold" and most of the legwork is done for you - the 3d is crunched down into net shapes based on whole objects. Sometimes pepakura doesnt think the way we do so you can tweak the parts using the seam and flap tools to make it more logical. If there are any red lines shown on the 3d side then that means that the points arent aligned properly and there will be a seam on the part. The only way i know to jigger this is to go back into the 3d modelling software and re-plot that point and the vertexs linked to it.(annoying O_o)
You can also rotate and englarge the project to best fit an a4 peice. If the parts are going to be too small and fiddly at this scale you can always enlarge so it fits more than one piece of paper but aim not to have the parts straddling an edge.
Once you are all sorted with this you can add in textures (which i am a bit hopeless with) and finally move onto printing. If you are going to paint your toy then it is best to turn off the textures (in the "both windows" tab), leaving just the phite of the paper.
The Shareware version of pepakura designer does not allow you to save your projects, meaning that you have to unfold the model every time you want to print off a copy and the part and flap arrangement is different every time O_o. However there is a function that isnt locked that allows you to copy the 2d page view onto the clipboard, so you can paste it into a graphics program and save it that way. This is a "quick fix" solution as you can then further edit it in photoshop or whathaveyou but will have to go throught the intital steps if you want to make any major changes.
If you are wanting to use this program a lot i would suggest splashing out and buying a key. Its not much and its a way of saying thanks (and i am in no way endorsed by the makers of pepakura designer, i just have ethics every once in a while :p)
Step 5: Get Crafty
Apply glue/tape and start constructing, work with the larger parts first as you get hang with it and they are more forgiving. Having a toothpick on hand helps as you can use it to rech inside the model and push the flaps for a better bond.
What i havent done is mark where the different parts join together, but if you made this sucker on the computer, your bound to know where the bits fit. If it is bothering you, you can always add them in on the "saved" version in photoshop.
If its not perfect then all is not lost, it just has "character" or "charm" (both nice ways of saying it went a little tits up O_o)
You can also mod your toy outside of the computer. Add paint, sticky out bits (resisting the urge to use the word "flange" :p) or even bits of tech jiggery. I am planning on making a Mushmech with speakers integrated into it at some point. (see DA)
If you have the urge "to go bigger" then photocopy your plans onto acteate and blow them up with an OHP onto mountboard or something equally meaty. Jus make sure you have all the parts to the same scale (check with a ruler so you have the OHP the same distance away from the wall each time). This happened to me during A levels when i wanted to make a human(ish) sized gundam. (last pictures, plans by Mad Modeller Masamune (who i cant seem to find a link to a working site for)) Learn from my mistakes and use a gluegun for anything of that scale as tape fails after a while.
Step 6: Further Perusal
Space Station 42 - site with lots of external links to various paper *things* all over the place. Sadly a lot of the robot ones are borked u_u
Papercraft world - bloggage of new hotness
Custom Paper Toys - excellent site where a guy makes a free model every months. A lot of them are automata (ie with handles and whirly bits and general awesome)
Flying Pig Toys - english site that sells paper automata kits. They also have a modest free section too. Muy bueno.
Shin Tanaka - Graphic artist who has an amazing line of paper toys. Lots of collaborations with big names on the Graff scene. heck, he even did some stuff for nike.
Readymech - Stylin' paper toys
Paper Robots 1999 - Poseable transformer action!
Fiddler's Green - Thanks amakerguy for showing this great website for making planes and scenery.
Also worth having a peruse at are Hako Clones - started by Masamune Washington, these are blocky versions of... pretty much anything. I've seen Stargate, Star Trek, Anime, Powerpuff Girls.... My suggestion is to just google it and see what the net provides.
There is also a commmuntiy on Deviantart (well 2 of us last count :p) that dabbles in the dark side of paper. Check under the origami folder though we use knives. AOUAL and my own stuff can also be found on there.
Have fun and may you keep your fingers out of the trimming zone.
*edit* I've just come across a website that lets you create your own paper models in a flash app- only head, body and legs but you can customise even more. Have a look at Paper Critters
**edit some more** Nice Paper Toys - An amazing grassroots community of paper modellers, verging more off towards 'urban' design but brilliant nonetheless