Pick a core such as: Dragon-Baton Pass sweep, Toxic/entry hazard stall, weather, etc.
From there, these are the typical 'essentials' of most teams:
- Sweeper
- Entry hazards or Magic Bounce
- Defensive/Sp. Defensive wall
- Perfect offensive type coverage (unless using Dragon offense)
- Offensive wallbreaker
And these are luxuries:
- Rapid Spinner (can also be Entry Hazards) or Magic Bounce
- Setup sweeper
- Stat boost Baton Passer
- Choice Scarf revenge killer
- Suicide entry hazard lead
From there, just build defensively around your core members' weaknesses and try to account for offensive type coverage. These are just the basics though, there's a lot more to team building than I feel like writing here, such as manipulating EVs other than 252/252 to fill certain niches among others.