Personally, I like a lot of carbonation so go with 7 grams per litre. That's with straight dextrose.
Carb drops are probably the easiest way. Many people dissolve the sugar in a small amount of water, then rack their beer to a bottling bucket, adding the dissolved sugar solution, then bottling from there.
Personally, the method I use is to dissolve the sugar in water, then use a syringe to add 20 ml to every 500 ml bottle. If you have 40 bottles (20 litres), you'll need to dissolve 280 grams of sugar in 800 ml of water....
Ummmm. Unless my rubbish maths has let me down, isn't it 140g for 20 litres at 7g per litre? I wouldn't like to use glass bottles with 280g of sugar priming a 20 litre batch as that sounds like 40 potential bottle bombs!