Draw these shapes very lightly, so that you can erase and correct them easily. And be sure to do just that until they look good.
Also add holes into the letters who need them (A, B, and D).
Note that the hole in the D is not perfectly round, but flattened on the left side. It looks just like a smaller D. This makes it easier to recognize the D for what it is. With a circular hole inside it may be falsely considered as an O.
Erase the inner lines afterwards so that only the border of the bubble letters remains.
Draw brighter stripes along one side of the letter, as if the letter was reflecting light at these places. Then draw a darker stripe on the opposite side, as if it lies a in shadow. This is quickly done and makes the letters much more attractive.
So these were the steps how to draw bubble letters. Below I show you an example how you can construct the other letters of the alphabet in the same way. I just cheated with the J, because its lower end is not an oval, but a curved shape.
Since the first five letters are shown above already it starts with an F here.
And here is what these letters look like after being traced and colored: