[Desire HD Q&A, Help & Troubleshooting] Teichopsia:
I'll start from the top.

Do you have a desire hd or an Inspire?

I'm assuming you are not rooted. To find out if you are rooted, make sure "fast boot" is switched of somewhere in settings when you are in the rom. It would either be under "battery" or "developer options" or one of those.

Switch the phone off. Hold the down volume button, and while holding it down then press the power button - wait, it should then boot into hboot.

In hboot it will tell you if you are s-off or s-on.

If you are s-on, you will need to root your device. If it is the desire hd, it's easier than the inspire (if you have the lattest software update on the inspire). You would go to aahk (ace advance hack kit) website which you can find in the thread I gave you. Take your time to read everything before you even download the hack kit. With the desire hd, it's pretty much a couple of clicks and you are done. With the inspire there are a few more steps - you'll be able to find all the instructions in that website. Make sure you read everything, including the effen manual. Read for comprehension. Don't skim it.

Now, if you are s-off, then all you will need to do is flash a recovery. You can find the instructions for that in the guide in post#7 as suggested previously [As well, the guide will give you a good idea on the terminology, steps, troubleshooters, etc., so that you can comunicate what is going on with your device in case you run into any problems and need help]. You can either use the method in the guide to flash a recovery or look through the second post which has a different method to flashing a recovery. Use whichever you are more comfortable with. However, if you are s-on, you should use the hack kit which will do everything for you except flash a different rom.

The other method is using htcdev to unlock your bootloader which will let you flash the rom.img. In the second post of the same thread, you can find a link on how to get this done. I won't go into how to flash a rom with this method, but the info is in that post.

If you use the hack kit, go on over to the developer section and search for what rom you would like to flash. As a rule, read the first page and the last five to have a good idea. In the past I used to suggest read the first five pages and the last ten, but as of late the dev section has gotten so overcrowded with junk, the same rule doesn't apply anymore.

Once you have read the OP (original post/first page), see what you would need to flash to get a working rom. Depending on which recovery you are using, CWM which comes standard when you use the hack kit (the wipe procedure is in the guide), or you can use a different recovery as 4ext (wipe procedure is also in the guide / and they are usually in the OP of each thread in the dev section). As a suggestion, try to get a touch recovery if you are going to be changing roms frequently. It will help spare the use of the hardware buttons to get things done.

Now, once you have flashed (installed) a new rom onto your phone, keep in mind that almost any question you can think of has already been asked countless of times. There is a search function which can be used to find whatever issue you are having. I can't recall if "how to use the search function" is linked in the thread or not. But how to report a bug is.

Hope this helps to get you started. Good luck.