Different mailbox types have different sending limits i.e gmail is 500 emails a day. You can update your batch email settings by going to Setup > select your mailbox > then go into Edit settings. Here you'll be presented with a drop down with options on batch sending timings to pick from. Select which one is most suitable to your needs and mailbox limits, and then click "Update Mailbox" at the bottom.
Before you start you'll need to create an email template and associated it to a project.
Select your contacts by ticking the box next to their name. You can use the the top box in the header column to bulk select all of them.