You get a simple sort of expander tool with each set or you can buy a 'plier like' tool. I have the plier tool and use it to gently expand the 'eye' until I get some contact with the bore. I then add some abrasive and start lapping using a little light machine oil. If there is obviously no more contact I use the pliers to expand the eye a little more until I reach my target diameter (in the case of my injector components measured with a precision pin gage).
As with all new techniques there is a learning curve, I put quite a lot of effort in learning how to ream properly
. Give it a try first on a hole in a piece of spare material.