Short answer: by looking first at my bills and then at my checkbook
Complete answer: if it's not covered by the above short answer, then it is simply some combination of (a) think of work I want to do (with "want" and sometimes defined by "need") and (b) get started whether I'm motivated or not.
Waiting for motivation is usually a long, long, disappointing and frustrating wait. Pick up your pen, or your hammer and saw, or whatever your tools are, and just get started. Force yourself to do it merely mechanically, if you have to. After a few minutes, it gets easier, and after a half-hour or so, you find you don't want to stop.
"I couldn't wait for success, so I went ahead without it." - Jonathan Winters