It is listed at the top of the thread, but here's the clipped version:
Originally Posted by novox77
This method can be used to flash to any partition, not just recovery. Essentially you place a file at the top (root) level directory of your SD card and HBOOT will automatically find the file and flash its contents. The filename needs to be named PG86IMG.ZIP for the Evo 3D, and it needs to contain the .img files named the same as the partition name.
Most of the time, you don't need to worry about all this; the file you download will already be in the correct format. You just have to rename it to PG86IMG.ZIP. The link to TWRP recovery above is distributed in this update.zip format. All you need to do is rename it and put it in the root of your SD card. Then boot into HBOOT. HBOOT will automatically scan for the file, and if it was named correctly, it will ask you if you want to flash what's in the file. Say yes, and it will do it's thing. That's it.
If you only have the .img file, you can make your own update.zip file. For example, if you have a recovery image file, it needs to be named recovery.img. Then, put it in a ZIP file without compression. Name the file PG86IMG.ZIP, and you're all set.
Take that file, rename it to PG86IMG.zip, put it in your SD card (not in a sub folder) and boot into the bootloader. It'll ask you if you want to flash the file it detected. You do. Leave it alone until finishes. IIRC, it's pretty quick (10-20 seconds). Reboot and delete PG86IMG.zip off your SD card.
Then check out the Error 16 thread.