For ease, I usually dose the whole tank at about 1mg/litre, so work out your tank volume, work out how many tablets you need (I assume you have 200 or 400 mg tablets, so for example a 200 litre tank would need 10 x 200mg tablets or 5 x 400 mg tablets) and just crush them into a glass (I squeeze them between two spoons until they turn to powder) then add some water, swish it about and pour it on top of the water - that's it. Some dust will settle on the bottom - don't worry, that's just caking agent used to stick the drugs together). Wait 3 days, do a 25% water change, and repeat. I work on the theory that although the fish doesn't ingest the drug directly, it is absorbed through the gills into the bloodstream when water laden with it passes through. It seems to work.
Alternatively you can crush it into / onto food, and try and direct feed it, which should be more effective (especially with agressive eaters like angels)