Lay out the text. After the comic has been blocked out and drawn, the letterer sets to work. First, put in the text how you want it to appear. Make sure everything will fit before drawing the bubble itself. On a paper comic, use pencil to lay things out, in case you need to erase them.
Edit the text. Make any additions, such as asterisks for explanation, or breath marks if your character is coughing. Breath marks, sometimes called "cat's whiskers" look like whiskers popping out of the word "koff" or simply around a space to denote a sharp inhalation. If you want to emphasize certain words, make them bold--but choose wisely, and try saying the line aloud to make sure the word makes sense emphasized.
Draw a circle around the text. On paper, draw the circle in pencil. If you're working on a digital comic, fill the circle with white and make sure it's behind the text. Squash the circle to somewhere between an oval and a rounded rectangle, as though looking through a TV screen. You may need to try a few circles before it looks right. The size of the balloon is also important. If the balloon is considerably larger than the text, it will look like the character is muttering. Generally, the balloon should wrap tightly around the text.
Draw the tail. Point the tail at the head or, ideally, the mouth of the character. Most comics have the tail ending about halfway between the balloon and the character, as though there's an invisible line continuing the rest of the way. If you point it in their general direction, it might look like their hand or thigh is talking, instead of their face.
If the character speaking is "off camera" or out of the frame, bring the tail to the edge of the frame. Make it so the tail doesn't end in a point, as though the point is also out of the shot.
Alter the edge of the balloon if you're going to. If the character is speaking, then a smooth edge is the norm; however, if they're fading out or dying, the edges may become wavy to denote that. A character's inner monologue usually has a bubbly thought balloon, and the tail is replaced by smaller bubbles. Musical notes or star-shaped balloons will indicate that a song is playing, or that the speech is coming from an electronic source, respectively.