How does a hydroboration mechanism actually work?
I had to write the complete mechanism for you so that you can understand it better. See the image below. I have explained it in the simplest form. Some of the points I would like to discuss before you look at the mechanism are:


In BH3, Boron is less electronegative than H (i.e. hydrogen) , hence in the syn addition I have shown, hydrogen ends up being attached to a more substituted carbon atom.
For every bonding orbital we have an anti-bonding orbital which lies exactly opposite to the bonding orbital and each bonding-antibonding orbital pair can contain a maximum of 2 electrons in them. 


Ok now lets head towards the mechanism which I wrote exclusively for you:







If you have any further doubts in the mechanism then you can ask in the comment section below my answer. I hope it helps.