I did a detailed post about this maybe 4-5 months ago.  I searched for a minute but couldn't find it.  Do a search for harley riser mod using a little more patience than I have this morning and you'll find it.  

The harley risers use either a 1/2"x13 thread or a 1/2"x20 thread.  They should come with the proper bolts.  If not, you can go to a hardware store and get some.  Get gade 8 bolts if you have to go buy some.

Also, besides drilling, you'll also need to remove the metal sleeve that sits inside of the rubber bushings to allow the 1/2" bolt to fit.  However, it's not a completely snug fit between the new 1/2" bolt and the inside of the rubber bushings.  It's close, but filling in that little space will add stability.  I took some aluminum flashing that is about at thick as soda can aluminum and wrapped it around the bolt in place of the stock sleeve to make a snug fit and maximize stability.  

Mine started out with these 6" risers.


Then I added these extensions along with this supporting plate that doubles as a mount for the raised headlight supporting bolt.


This is the final product.  This pic was one I used to highlight the new air filter but you can get an idea of where the bar is with 8" riser height.