Take your starting 1st level character, and let them pick a class as normal, with no racial benefits or penalties.
At second level, they gain the racial abilities of the Aasimar-- but no class level, HD, or skills.
From third through fifth level, use the Aasimar to Half-Celestial transition class.
If you're doing this for an established character, I'd recommend making the transition to Aasimar cost two levels (since they already have racial traits). Give them the special abilities first, and the ability score adjustments second-- again, with no class levels. This is essentially building up to the LA.
I'd save the change to Outsider type for the third level of the Transition class.
