Here is one way to do it. Yesterday I decided I would walk on a treadmill for 30 minutes. Well, on the way to the fitness center, my mind was already saying, "Oh, just 20 minutes will be OK." And then I started on the treadmill, and my mind was saying, "This feels hard today, maybe I should only do 15 minutes." But I had decided that I would do 30 minutes, right? So, I just kept going with it.
Did my mind stop its chattering? Oh no. I got to 20 minutes and my mind told me, "It's OK, you've done enough." I kept going. At 25 minutes, my mind said, "This is really close enough to 30 minutes." I kept going. At 29 minutes and 30 seconds, my mind told me, "You really are crazy, you can just stop now, what difference does 30 more seconds make?" I kept going.
So the point of my little story is that sometimes the mind is not helpful because we try to let ourselves "off the hook." Don't listen to those stories about how tired you are, how much you don't want to do it, how cold it is outside,
Just start, and then finish!