How to draw bubble letters by hand
Write the word in thin, widely spaced capitals first. Draw a soft rounded outline around each stroke at an even distance, letting the outlines of neighbouring letters touch or overlap slightly. Erase the skeleton line, then add a highlight curve in the upper left of each letter and a drop shadow on the lower right to make them look inflated.
Letters with enclosed spaces (A, B, D, O, P, Q, R) need a small inner bubble so the hole does not disappear; keep it open, because shrinking it is what makes bubble letters hard to read.