Draw the domed cap of the mushroom and flare it out at the bottom. Make it fat and stumpy because kawaii cartoon characters are often short and round.
Create the stem of the mushroom directly under the middle of the cap and make sure it is slightly thinner at the top. Flare it out as it nears the bottom. Draw the stem so that it is about twice the height of the cap. Make one side of the bottom of the stem about 1/4 of the way from the left side of the cap and the end about 1/4 of the way from the right end of the cap, so that the bottom of the stem is about half as wide as the cap. The bottom of the stem should round out on the sides and not stop flat.
Draw the outlines of two eyes in perfect circles, about 2/3 of the way up from the bottom of the stem. Center them and make sure they are two portion widths apart. In other words, you should be able to fit two of the same sized eyes between them.
Add the outlines of two identical round shapes in each eye. You don't need to make these shapes perfectly round because they are the reflections of light in the character's eyes. Fill the rest of the eye in with black. Draw a rounded lower case "w" below and between the eyes for the mouth.
Create circles in different sizes on the cap of the mushroom to look like polka dots. Now that you have the lines drawn, you can add shading and coloring if you want to complete the kawaii mushroom.