The general principle is to use a mirror or piece of glass as your chopping board. Place your tablet in the centre and gently crush it using the back of a spoon. Keep working at any lumps until only fine powder is left. I don't know how easily subs crush down, but if you can't get it fine enough using a spoon then you'll need a Stanley knife blade or similar to chop up any course lumps. Take a bank card and use one edge of it to scrape all the powder together. Then use forward and backward sweeping motions to draw the powder out into thin lines - the thinner the better really. This can be tricky at first, so just play around with it for a while to get the hang of it. If you fuck it up, just scrape it all back into a pile and start again. Get a clean sheet of paper approximately the same size as a £5 note (it's traditional to just use a banknote, but that's not really advisable) and roll it tightly around a pencil to make a tube. Slide the tube off the pencil and keep hold of it to stop it unrolling. Push one end of the tube up one nostril as far as it will go, taking care not to scrape the inside of your nostril with the sharp edge. Now, assuming you're right handed, you want the tube in your left nostril. With your right hand thumb and middle finger, hold the tube in position in your left nostril while using your right hand forefinger to shut your right nostril off. Now breath out through your mouth then lower your head down toward one end of one of the lines until the bottom end of the tube is directly above, but not touching, the powder. Keep your right nostril tightly closed and simultaneously sniff up hard through the left nostril and follow the rapidly disappearing powder with the tube end (and remember not to blow down it). It's often best to do a line up each nostril to equalise any discomfort. It might take a bit of practise to get it right, but you'll soon get the hang of it.
But just to reiterate, I really wouldn't advise you to do this at all...
