I bought the plans from flamethrowerplans.com for about $50 about 6 years ago. The ebook is now $97. Over the years I upgraded to some of the parts used on the X15 Throwflame system.
A good unit can be made for about $500. If you really want to make one, I recommend supporting the flamethrowerplans guy as he has the cheapest, fastest plans. Between the throwflame and flamethrowerplands videos, there are many helpful video tutorials on fuels.
Update:
I just wanted to give Rob @ Flamethrowerplans.com a big thank you!
I bought his ebook back in Dec 2012. I just emailed him to see if he made any improvements to his system since I bought them. He just sent back his updated plans free. It is full of suppliers links, pics, step-by-step illustrations, tech and FAQs . That was very nice of him.
His plans cost $97 and are worth it. It saved me a lot of R&D in the beginning. I recommend his plans.
I do have some suggestions to improve the ease and cost of his system as compared to the Throwflame X15.
1. Only get one Bernsomatic torch and do not modify it. It can be mounted just like the Throwflame X15 system. Just use the 1" 1/8 NPT pipe on the muzzle instead of the 3" nipple.
2. Do not buy the extended propane torch hose. Save a lot of time and money. Mount the torch head to the power wash wand like the X15. Screw the propane tank to directly to the Bernsomatic cast torch head like the X15. Keeping a propane torch tank on it's side has problems with gas delivery.
3. Do not use the quick connect fittings on the 3/8 hose. They leak. It is easy enough to unscrew the newer power washer wands from the hose by hand.
4. His cheap scuba tank backpack is much easier to use than to make one like the X15.
5. I also prefer the USGI 2 grenade/3 mag pouch to hold the 20 oz CO2 tank, the extended ASA hose and 2" pressure gauge. The cooling CO2 tank can chill two beer cans that hare held in the grenade holders.
6. Buy the $50 Loews power washer wand.