It's straight forward enough - if there's fluid in the washer bottle try to siphon it out first; disconnect the pump wiring plugs pulling upwards; either disconnect pipes from the pumps (you need to turn the collars 90 degrees or so) or if you've emptied the washer bottle you can pull the pumps upward and outward to release them from the clips on the washer bottle; between the inner wing and washer bottle there is tab on the washer bottle that clips it in so lever that off and lift the bottle upwards; the fluid level indicator switch underneath needs to be turned 90 degrees to allow it to be pulled off the washer bottle.
The pumps just push into rubber seals each of which have an integral filter and it's usually these filters which clog up - you can renew them for a few pounds or simply clean them to remove the sludge that can build up in the bottle. Also get any sludge in the bottle washed out - I recently did mine with a few dishwasher tablets in hot water and left the washer bottle immersed in a bucket to soak overnight, it only took a little bit of brushing to catch the last remnants of sludge.
Wisdom on this and many other BMW forums suggest the sludge forms as a result of mixing BMW screen wash fluid with generic alternatives.