I leveled two characters with WvW only. It’s a good way to avoid the complete boredom of PvE leveling and the very high cost of craft leveling, and you can also learn quite a bit about how to play your class in WvW while doing it. Here are some common sense tips:
1. The fastest way is roaming the small stuff in a small groups constantly hitting sentries, yaks, camps, and the small PvE stuff like harpies and wurm. All out karma trains during the late week is also fairly fast but, more boring.
2. Buy new gear every 10 levels. Green full set of armor, full set of trinkets, weapons, and inexpensive gemstones to put on armor and sigils for weapons. Low level food / oil is also cheap and gives extra XP. This should cost no more than 1g total per set with instant buy, and it actually gives you a major advantage the first 3-4 levels with similar stats to an 80 with exotic. Don’t be an idiot upscale that gets instagibbed by anything. Avoid hammer trains, have defensive skills for periphery gankers.
3. Play an 80 that is appropriately geared for WvW when it’s somewhat serious fighting. You can’t always hump siege. Even if you follow the previous step, and are a good player that’s actually helping more with your upscale than most idiots in WvW, it really annoys commanders when they see so many upscales and are losing battles. Karma trains on the other hand, everyone knows those are basically farms for karma, champ bags, WXP, and XP and it doesn’t matter if you have a level 2 ranger or an 80 gold rank zerg warrior.