You do not need to force slang, but knowing common words makes interactions warmer. Use them lightly and listen more than you perform.
Best time
Any casual interaction.
Dress / vibe
No costume. Just be normal.
Local tip
Move like you know
Start with greetings. Do not overdo slang you do not understand.
What to know
Sasa means hi or what's up.
Poa means cool.
Msee can mean person or dude depending on context.
Soft safety note
Language is context. If unsure, plain polite English or Swahili is fine.