There is a circular guide tool in the Medibang Paint comic drawing app. And I use this to draw my word balloons.

Selecting the circle guide

So with the drawing tool selected, I can go to the top menu and choose this circle guide button here.

After the circle guide appears on the screen I can move it and size it the way I want.

Drawing the word balloon

Then I can draw an outline on the circle guide and now that I have an oval that's above my character's head I select the erase tool and remove a section of it to draw the balloon tail. And now I've drawn a little word bubble.

(Related Post: A Beginner's Guide to Medibang Paint For Webcomics)

Now I just need to color it in so I go to the color layer, pick the bucket fill tool, select white for the color, and fill in the word balloon.

Now I can add text to that.