You will need: A ruler or tape measure, tailors chalk or some other non-permanent way of marking your tights, small sharp scissors and a pair of old thick tights or new ones if you're brave (mine are at least 60 denier). The thicker they are the less likely they are to ladder once you've slashed them. Leggings would work too.
Put your tights on.
Mark lines across the middle of your leg where you want to make the slashes. I started mine at the thigh just low enough to be decent and marked lines 4cm (2") across with a 1cm (1/2") gap between each line. With hindsight I think this was a bit wide but it's totally up to you how wide you make your slashes and how far apart they are. It's not an exact process I really didn't do mine all that neatly. In fact I gave up using the ruler half way down my first leg and as you can see not all my slashed are the same length or the same length apart but it still works.
Cut along you lines. Be careful not to cut or scrape your legs while cutting (like I did).
Ta Da! Done.
You could use some clear nail varnish around the edges of the slashes to make them more durable but I didn't bother. Although note that I haven't actually worn these out yet. I have no idea how long they will last.
Slashes can be worn at the front...
...or at the back.
Personally I prefer them at the back. From the front you are just wearing plain black tights then turn around and suddenly it's a very different look.