i suggest not using rom manager,as its an APP that requires a working rom in order to be available to you. if you have a bad flash,wich is no biggie,its happend to all of us,it can leave you hung at a splash screen,in a bootloop,or into a non-working rom with no market and no way to side-load apps
this can cause quite a moment of panic if rom manager is the only thing that you know how to use. ive seen the painced posts in many forums during my time here.
rather than use rom manager,you will use Amon or clockwork recovery.i personally prefer amon,but the choice is yours,and either should accomplish what you need it to.
your permanently installed recovery(amon or clockwork) will ALWAYs be there for you. in a working rom,you can use the "boot to recovery" option in an app called quickboot,or you can even use the "boot to recovery" option in *cough* rom manager. if you get stuck with a non booting rom,just pull the battery,boot to hboot via power/vol down,and then after the green writing flashes,select "recovery" with the volume rocker and power button.
a good first step would be to simply go to recovery somehow(either of the methods above) and explore the menus. see if you can figure out how to make a nandroid backup. you can then search or ask us specific questions about manuvering in recovery.
last and not least,there is a "new thread" button at the top left,above the sticky threads at the top of the forum. click that,and ask away
if you use clockwork,youll use the optic trac-thingy to highlight and select various options. the power button does NOT select,it takes you back a menu. if you happen to be at the main menu,it takes you to a screen with the rom manager logo(the hat). this has caused alot of folks some confusion