The surest way to get rich slowly is to save.
If you start young and you set aside each day or each month a small amount of money from your income (most likely a job), in 40 or so years you should have amassed a small fortune.
Of course, for this method to work you have to start young, you have to have a steady, large enough income, and you have to be willing to live a bit below your means.
This method of getting rich slowly can work if you have a clear goal you want to achieve in your lifetime with those savings. If you don't, then all those years that money will be sitting there doing nothing - pretty much the same as if you didn't have it all.
Although the surest way to get rich slowly is to save, that still doesn't mean it is sure enough - in the global world 40 years is a very long time in which the world can change many times over.
For most of us it makes sense to find faster ways to get rich instead of wasting our time (our most valuable resource) with such a long wait.