First, if you need help:
http://151chan.org/truck/res/198.html#i198
It's good to know this too, You'll be running a lot:
http://151chan.org/truck/res/200.html#i200
As a side note, it's a BAD IDEA to rack up your steps BEFORE you drop a pokemon into the walker, if you intend to use your own. You will lose all watts. More on that in a bit.
Of course, you'll need Yellow Forest unlocked. Until May 3rd, there is a Wireless event to get it. Simply connect your DS to a wireless connection and choose 'Mystery Gift'. After you receive the route from the man in green from any mart or dept store, DON'T FORGET TO SAVE!!! It won't do it automatically!
Now, you won't be getting flying pikachus unless you have 10,000 steps. A lot, I know, but according to Bulbapedia, if you have a pokemon with an advantageous type, that number is reduced by 25% That makes it a mere 7,500 steps. (Or whopping 7,500 depending on your perspective.) In this case, Electric has an advantage, so simply drop one into your walker or wait for a Pikachu to join you. Given that 20 steps equal a watt, that means if you go the long way with 10k steps and lose all your watts by dropping a poke into your walker AFTER you did your steps, than you've just lost 500 Watts! Congratulations! Now do it all over again. (See? I told you I'd get back to that, I didn't forget :P)

Now, for the 'meat' of the quide. All that reading was just the set-up. At this point, you should have an Electric Pokemon, probably Pikachu, and 375 Watts with 7,500 steps, or a non-advantageous type poke with 500 watts and 10k steps.
Now, to the Radar! Find a pokemon. If IT WAS JUST ONE ! OR A SECOND ! BUT NOT !! OR !!!, just evade untll it flees. This pokemon will only be a class C (crap) or a Class B(Not Crap, but not worth risking either.) You're far more likely to run into !!s than !!!s, so it might be a good idea to catch 3 of them. (!!s, I mean). Those have a chance of either being class B (cool!) OR Class A (Awesome!) Class A have either Surfing or Flying Pikachus, which is what we're going for. Therefore, don't catch anything that isn't !! or !!!
Obviously, if you encounter a !!!, it will always be class A, so catch it! Catch the living shit out of it!

Now, if you have 3 pokemon already captured (not including the one walking with you of course) you won't be able to directly tell which Pikachu is which. If you pay attention on other pokewalker routes you can surmise how each of them are stored, but I don't have that info. So, if you don't know, what I'll do is replace the FIRST pokeball, and then the SECOND pokeball. This way, if the raplacement is put on your choice OR if it is placed at the end, you won't have to worry about replacing a !!! pokemon.
Clear as mud? Okay.
Finally, it's up to you to chance !!s and not spend as many watts on pokemon so you can get a coveted light ball or 2 from Dowsing, or go for only !!!s and quarantee you'll get a surfing or flying pikachu, and run out of watts quickly. :P Naturally, you can simply cheat your walker for more as mentioned.

If there's any questions, feel free to post them. As I rarely actually look at the site I can almost guarantee it won't be read, I promise.