Depends how experienced you are.
I picked a single Pokémon I wanted to use and built around it.
As you battle, you'll notice certain threats you can't stop or that some Pokémon on your team aren't pulling their weight. Adjust accordingly.
And it never stops. You can always make small improvements.
Plus the metaagame changes. Added Clefable to your team to stop M-Sableye? Smogon might ban it.