First things first. You need root access for a detailed list of instructions check here: How to "root" the LG Ally
An easier way of doing this is by getting "universal androot" from the market.
Next you'll need an application called Rom Manager, this is available through the market.
Once you have root and have Rom Manager installed it's fairly easy.
1: Flash Clockwork Recovery in Rom Manager
2: Once flashed make a back up, make a back up, I can't stress it enough, make a back up!!
3: Using your computer download Velocity 0.3, here's the link: goo.gl/VnXo
4: Once downloaded move to your phones sd card, don't put it in any kind of folder, just leave it in the root directory of the sd card (ie. e:/velocity_v0.3_release.zip where e: is your sd card)
5: After it's been copied to your sd card go back into Rom Manager and select Reboot Into Recovery, this will reboot your phone into recovery mode.
6: Using your physical D pad with the keyboard slid out, select these options, wipe data/factory reset, wipe cache partition, advanced, wipe dalvik cahce.
7: Next you hit the back button on the keyboard and select from the recovery menu "install zip from sd card", select the velocity 0.3.zip file and watch it go.
8: After it reboots and loads up, the verizon logo splash screen goes, then a cool new VELOCITY animation till it gets to your home screen, VIOLA, you're now running Velocity 0.3.
Hope this helps, let me know if there is a step I'm missing, I'm 99.8% sure that's that all you need though.