Whether it be to do your job, get your schoolwork done, clean your house, work on your creative passion, etc.
Bonus points if you’re someone without reasonable self-control so saying something like “I can only have a cookie after I do this/while I do this” doesn’t work for you and you’re just going to eat the cookie and not do it.
I don’t think, I just do.
I usually keep an end goal in sight; for example, I sometimes dread going to the gym, but I always remember that my goal is to stay fit, have a healthy body and exercising is an important part of that. By sticking to my goal, I maintain my discipline and go to the gym 3 times per week.
Another example is school homework; in my case, my math homework is something I don’t enjoy, but I remind myself that I need the math certificate in order to enroll for a Computer Science degree at an university. Therefore, I keep pushing myself to study math and get good grades (which so far has worked pretty well)