Harvester Walking Guide
Harv(ester) Walking, you've probably heard of it and maybe even tried it, but not too many know how it's supposed to be done. Basically harv walking is walking behind the harvester with some infantry unit to the enemy base without getting killed by the base defenses. The best unit to harv walk is obviously the Stealth Black Hand, simply because other units are pretty easy to spot. However, as long as full radar is off, it's still possible to do it with other units, and no one really expects GDI to harv walk because you've got to be a stupendous badass to even consider it. That'll give you the advantage of surprise, at least. Getting a beacon placed with an SBH on some key building on a base defense map just might win the game. Sneaking in with a technician/hotwire is great as well, since they can destroy a building on their you got owned
 Harv Walking
So, harv walking works on maps with base defenses. It's possible to harv walk on City, Mesa, Field, Under, Glacier and even Hourglass not to mention all the fan maps out there.
 The Basics
First, you might want to press 'F' for third person view so you can observe the situation better. Though it's sometimes better to have the first person view so you can definitely say if the base defenses will spot you. Wait for the harvester somewhere safe and go with it when it's going to return to dump its tiberium. Since you need to match your speed with the harvester, I recommend pressing and holding the jump key (default: space). This makes you 'jog' and go at a speed of a harvester...well, a little faster actually. As long as you're walking behind the harvester, it's OK to run at full speed and ride the harvester's back end, but if you're trying to go around it you want to match its speed as close as possible so the base defenses don't spot you. Try to keep the harvester between you and the base defenses. Like shown on this screenshot from Field.
Follow the harvester and depending on the map and where you want to go, follow a path. Here are a few paths on some maps.
You can get behind power plant, barracks, weapons factory and refinery easily. Just jog on the right side of the harvester and try to keep your eyes on the advanced guard tower.
This is how you should position yourself
At this point, start walking diagonally if you want to get to the PP. Just keep jogging on the right side of the harvester if you're heading for the Ref or WF. At about this point you're also safe behind the barracks, so you can just stop, wait for the harvester to go on and run to the barracks.
If you're going for the power plant, just keep looking at the AGT and jogging towards the PP.
At this point you should release space and run behind the power plant. If you do it correctly, the AGT won't notice you. This goes for all four buildings on this map.
Getting behind WF and ref is possible without getting shot at. The AGT will most likely get a shot (and a missile as well then) at you if you try to get to the AGT. It's possible to run from behind the refinery to the power plant and from there to the barracks. But usually it's enough to nuke either the war factory or the power plant.
Jog behind the harvester and when you get closer to the AGT, move to the side, like that.
Here's a picture of being at the side of the harvester.
At this point (if not a bit sooner) you should release the jump button and start to run. Keep on going and run to the front left of the harvester.
Keep on looking at the AGT and keep your distance to the harvester. You don't want to get run over by it at this point. When you see the harvester is going behind the weapons factory and the AGT can no longer shoot at you, run for cover and you're safe behind the weapons factory. No one saw you and you can plan your nuking point in peace.
Now, if you can master these two, you can proably make up your own routes to places. I'd suggest practicing on Field, as it's probably the easiest map to harv walk on.
===Advanced Techniques=== (uhm...not really serious here, but these are doable...just takes a lot of luck or not-so-bright enemies to pull one off) Walking behind a mammoth tank:
Mammywalk to AGT, nuke it and steal the mammoth and cover your nuke with it.
Harvwalk on Hourglass:
This obviously only works with a chem trooper. But, it's doable.
Also, consider harvdriving with a stealth tank. It's actually pretty easy to do the same things with a stealth tank as you would with an SBH. This has obvious advantages, such as not getting damaged on tiberium (Mesa, Hourglass) and you can't get driven over by the harvester. You can become a stealthy technician even. And if you do pull it off, you'll have a tank covering your nuke. Though bear in mind that the stealth tank is much wider than an SBH. This means you can't get everywhere an SBH would. Buggies or Light Tanks also work very well for this as they have a very low profile and are quick.
Also, harv walking is NOT the same as ob walking or ob glitching. Sorry for the picture heavy guide, but I think they're neccessary.
 Important Note
Harvester walking is against the rules on some servers and will get you kicked from the game. Please remember to always type the !rules command to check before you do it! You are expected to have read and understood the rules on most servers the moment you join. Also please remember not all rules are explicitly stated under the !rules command so alternatively you may wish to ask a moderator or check the server's website or forum. If in doubt, don't do it.
Example Videos (Right-Click and 'Save Target As' for best results.)