once your rooted you are going to have to download several things to get started
Rom manager
Rom manager Premium(Paid)
Droid X Bootstrap(Paid)
Premium and Bootstrap i do recommend its much easier than going into recovery mode and doing it that way, but its your call
with bootstrap and rom manager premium is as follows
1- once you have downloaded your rom from your pc, plug in your usb from computer to phone, then move it to the root of your sd card which is the first thing you see when you enable mass storage mode, just right click in a open white space and paste
2- once its done moving, unplug usb from computer
3- open Droid X Bootstrap hit bootstrap recovery
4- hit allow for root access, and wait for success
5- open rom manager after you have already downloaded rom manager premium
6- Back up current rom
7- Name it STOCK
8- Once phone boots back up go to Droid X Bootstrap
9- hit Bootstrap Recovery, and wait for success again
10- open rom manager
11- install rom from sd card
12- find your rom you want
13- hit it and wipe data/cache to be safe
14- always hit allow if it asks you
15- wait for phone to boot up
16- enjoy your new rom
hope this helps