In order for your harmonizations (written down or improvised) to sound as natural and logical as possible, there are some important rules and breaking these rules often leads to less than satisfactory results.
I have taken these points from the actual examples I see students make at M.K.Ciurlionis National School of Arts in Vilnius where I teach so you can be sure they all are practical and down-to-earth. Chances are you might be making them, too.
In reality these mistakes are not that difficult to avoid but you have to be aware of some of the methods and techniques that I talk about in this video.
I hope this video will be useful to you. After you watch it, take action and apply my tips in practice with the hymn or chorale tune of your choice.