Who are they?
The friend you can depend on and who loves you and can deal with a little bit of crazy. She is helpful and understanding, but also can be very determined to get things done.
She can make it on your wedding day (there is always one who is overseas or perhaps expecting to deliver a baby)
She says yes to helping you on your special day.
How will you ask?
It can be as simple as a call or text.
Better would be laying out a few of the details – how special they are to you, asking politely that they be a bridesmaid, what you are hoping they will do for you (buy a dress, attend the wedding, etc)
Nowadays there are themed cards, cookies, wine all around asking friends to be a bridesmaid. These are definitely cute and thoughtful ways to ask the question and pretty fun to consider so I encourage you to explore if you enjoy that kind of thing.
What happens next?
Connecting your bridesmaids and introducing them to each other. Let them know who else is in the group. Then they can get to know each other, possibly plan a bridal shower or bachelorette party, and generally be in touch. A group email, or text, or Facebook group can encourage intermingling.