It’s best to ask one person instead of a group. Pick someone who you think can help you the most, or at least send individual requests to several people at once instead of dropping a line in a group chat. 

Asking a group leads to the “diffusion of responsibility” phenomenon, where no...

... and be specific about what you want.

Don't use phrases like “Can you do me a favor? ", because they are manipulative - they force someone to commit before you tell them what it is you need. A simple “Can you help me with [specific thing]?” will do.

When you ask for help, give the person some kind of timeframe or soft deadline. Phrases like “whenever you can” put more pressure on the person who is already doing you a favor. 

It’s also nice to offer them a way out if you know they’re busy. It ensures your request for help ...

When you ask for help, you may feel inclined to apologize for taking up their time and energy. Don’t. 

This is a bad idea because putting yourself down makes the other person feel less joy in helping you. Many of us apologize too much anyway.

Just because someone didn’t help you before doesn’t mean they won’t help you now. 

In fact, research suggests they’re more likely to help. Because they want to feel better about rejecting you previously and repair the relationship.