Sow a habit and you reap a character.
Sow a character and you reap a destiny.
The man who said that was a pretty smart guy, because he knew the power of a habit. But let's start with the act.
Putting an act out there is often equivalent to "starting something". If you have not done something before, it is very hard to start. As we are in the middle of the turkey and chocolate loaded holiday season, we all know how hard it will be to start working out again after having gained a few pounds. It just doesn't feel right to start going (or wobbling...) to the gym again, although we really want to.
But it is hard. It is hard because we doubt that making an effort is even worth it anymore. We are discouraged by the fact that we were in much better shape a couple of weeks ago. So we are paralized because we have lost faith in what our actions can do. So restoring our faith is the first step in order to create that "act" that makes you harvest a destiny in the long run.
How can we accomplish that?
Try the following, staying in the sports after holiday example: Try to picture how you would feel if you went to the gym tomorrow, the day after tomorrow and again in three days. Three times in a row. How would you feel? Probably a little less wobbly. If you repeated the same thing the week after that, your perception of yourself will be even more positive. The whole point is that by only making an effort, our confidence rises significantly. Making an effort does not necessarily mean succeeding. But even failing in getting in shape in one day does not seem too bad anymore. Because we know that we are on track to get there. And this is how I interpret the quote from the beginning: It is about harvesting more than you have sown. You are not just doing something valuable for yourself - you start gaining confidence by doing it as a free extra!
And after that it gets even better: if you grow a habit of the acts that you have sown, formerly hard and scary things suddenly become natural and empowering. I asked a good friend of mine a couple of weeks ago, how he manages to literally be in the gym every evening to practice martial arts. He told me that for him, it was as natural as brushing your teeth in the evening - but more enjoyable. I think that this is a good place to be. If we can manage to practice something difficult so often that it becomes a part of yourself. Harvesting a character is then only a matter of time. Although I don't think that going to the gym makes us better people, the important idea is that we can indeed change what we do on a regular basis, thus changing what defines us.
That is why I find phrases like: "I just cannot do that!" highly questionable in theory, because I assume the person who says this has not really tried. In practice I think that I have no talent for playing instruments, without ever really having tried. So there is much of the above to implement.