Here are some tips on how you can do just that.

Do your homework and get to know the influencer before reaching out to them
Always start by appreciating their work before making a pitch
Clearly highlight what you’re offering in return for their effort
Get to-the-point, mention everything relevant, but don’t make your message too long
Be respectful and leave scope for further discussion and negotiation
Follow-up if they do not reply to your first message.