We have been breeding Highland Cattle since 1991 and I have tried various ways of removing the horns on Highland Cattle. 
There are several ways to do this and the preference of method is up to the individual. 
Different methods are as follows - 
As calves you can scollop the horn buds out with a tool at about 2 months old OR Burn them with either an electric debudder or a gas debudder  
It is all over in a couple of minutes and the calves recover very quickly.  
Older Cattle -  You can cut the horns with a saber saw, it seems to quarterise the horn and it doesn't bleed as much. Make sure you use the white fine metal cutting blade.  It is definately adviseable to do this in winter as the flies will be a problem in summer.   Unfortunately the rules for Health and Safety have become an issue in Australia and horns are not accepted in the yards and abbatoirs, it is regulated now in Victoria and some interstate, but will be mandatory in all other states within the next year or two. All horned cattle have to have their horns cut off below the end of their ears. 
There is a simpler way -  
              dehorn by using a poll highland bull
Poll Highland cattle society inc