What does "to flash" mean? Is it easy?
Flashing means to install a ROM over the one you currently have. It is something that will void your OS warranty, and potentially damage your phone.
I am unfamiliar with the steps and processes to flash a ROM on an HTC, but for my Samsungs the general steps are to 1. Gain Root Access to the device. 2. Flash a custom recovery (a non OEM recovery program) which allows you, the user, to do certain steps in the flashing process. 3. Installing the new ROM via a download mode or installing through recovery.
Rooting and Flashing can be easy and fun. BE ADVISED: this is also something to be very careful of. Always make sure the files you are downloading are an exact match for your device model. Using Root Access files or a ROM file that are not for your device can lead to bricking it (rendering the device useless like a paperweight or brick). Never install International ROMs to US carrier devices, and equally damaging is to flash a ROM from say Sprint to Verizon. If you have Verizon, ensure you are downloading Verizon files.
I am sure there are people who can help you in the One X forums. YouTube may have some good advice/videos as well. Since what you want to do is for convenience, I would highly suggest to take your time, study all procedures fully so that you know them forward and backward. Start with Rooting. It is generally easier than Flashing ROMs, and it's own benefit is being able to run apps that require root access. Remember: Read, read and READ all directions. Any questions, ask before doing anything.