The System
This represents your leveling mechanic and how exactly it works. A few ideas could be that anytime the unit kills its points worth of opponents then it should be eligible to level (a little unfair as it favors units that are equipped to take out heavy targets rather than those that deal with lighter foes) or as the old 40k rules mentioned, you could just assign a veterancy level to any unit that survived the match by the end of the game.Whatever system you choose, try to make it fair for every unit, vehicle and creature to level up. Some systems may favor others and some other systems may seem a bit eccentric, but come up with something that has clear boundaries to all players and try to make it intriguing without it becoming a burden (counting a certain number of kills or amount of points a unit killed could be difficult, but it could be a more realistic option than picking a survivor of a war that did nothing to become a master of his trade).
Applying It
Probably the trickiest part with an experience system in 40k isn't getting the system working, but applying it and figuring out what kind of bonuses a unit could receive. 5th edition suggested that you roll from a random table once you declared the Veteran unit and hope that it applied to the given unit (sometimes you would wind up with Furious charge on you Crisis Suits). Other ideas could be applying stat bonuses, but this could become a REALLY broken feature if you let it go on without checks.As I mentioned above, giving stat increases could be a really fun way to truly make a unit unique. Instead of it happening every level, make it every-so levels or certain stats. Some stats shouldn't be changeable (such as Sv and W) and of course the limit should be ten, but other than that decide on how often these units receive these skills points and watch some crazy things happen.
Don't forget to read the Heroes and Veterans section to fully understand my ideas on implementing this system to help integrate it and restrict it (you really don't want to have units running around with 10's across the line and almost every USR known to the Imperium).
If you have any questions, don't be afraid to leave them in the comments section (I realize that this was kind of vague, it's ultimately up to you and your group) or shoot me an e-mail if you really don't want to share it publically. As always, I'd be more than happy to help you and your fellow defenders/destroyers of worlds to set-up your own campaigns of conquest.