What sud says about native German-speakers simply plonking their umlauts over vowels is quite true. However, given that the result of a quickly drawn dot is often a nearly invisible smudge, it's usually represented differently in handwritten German. Often a simple horizontal line is drawn above the vowel, and it looks just like the macron that students of Latin will be familiar with (though of course with a very different meaning). Other times, two vertical lines, like a narrow equals sign rotated 90°, are drawn above the vowel—imagine the dots of the normal umlaut extended up into a line. Both of these representations of umlauts are easier to write and read than the two dots, and I find the vertical-line method quite beautiful.