check out the liberty sticky thread about how to install it put i can tell you here... download the rom and put it on the root of your sd card and then boot into clockwork using boostrapper then use the volume keys to get to backups and backup if you havent yet and then wipe data and cache click on install from sd, then click on liberty BAM! you got liberty!
There are detailed instructions in the very first post of the Liberty 1.5 release thread. Here is instructions per their thread....
make sure you have root and clockworks recovery installed
-boot into recovery via clockworks bootstrap
-select "wipe data/factory reset"
-select "Yes -- delete all data"
-go back "powerbutton"
-select "install zip from sdcard"
-select "choose zip from sdcard"
-find and select LibertyX/2-X.X.zip