1: Go to the Safari Zone, try to catch a male and a female Pikachu.
2: Fly to Mauville and head west to the Day-Care. Put both Pikachu in. Make sure both of them arn't holding anything.
3: Get your Mach Bike out (Switch if you don't have it.) Ride From the coast on Route 118 to Verdenurf and back until the old man at the Day-Care is standing outside the fence. Retrieve your egg!
4: Take both Pikachu out of the Day-Care immediatly after you get your egg. If you don't need them, put them in your box.
5: Go on doing normal routine until the egg hatches.
6: When it hatches, it should be a Pichu that knows the following moves:

Thunderbolt
Charm
Double Team
Tunderbolt

7: Enjoy your super strong, super cute Pichu!