It's a very easy process. Just get there nice and early. You want to know the route that monks take on their alms round so you can find them. Alternatively, you can just wait outside a monastery as you will be able to see them. Any food you wish to give is fine, but give the best you can. Simply stand there (there will likely be many other people doing the same) and when the monk comes up, give alms. They will know what you are there for.
Whatever an enemy might do to an enemy, or a foe to a foe, the ill-directed mind can do to you even worse.