Lilies are bulbs and, as such, can't be 'split'. Each bulb produces one flowering stem so, if you have a number of stems, the bulb has multiplied itself and the separate bulbs can be divided off.
Wait until the stems have died down, September time, and cut them off at ground level, then dig up the bulbs being careful as they can make quite a large plate. Brush off the soil so that you can see what you are doing and then carefully prise the outer bulbs away from the inner ones. Replant them straight away.

Unless they are Arum Lilies (sometimes known as Calla Lilies) which grow from rhizomes rather than bulbs and can be split.

I usually put a fork in deep and lever up all round the clump to loosen the soil and then use two forks back to back in the middle of the clump to lever apart.

Hope this helps :)

Farmergeddon? Wow, hope you don't live up to that!!

Alas Steragram, I did choose that name due to being prone to garden disasters :D
I've had plenty of successes too - but there's no middle ground with me it seems.
I'm either snowed under with stupid amounts of tomatoes or I lose the lot to blight kind of scenarios :-/