the quick answer is "carefully"
you can buy screen cleaner - or screen wipes
but the DIY approach is simple enough
you just need to be gentle:
a soft cloth - not paper towel or a panscrub
natural soap - not detergent (actually 1 drop of shampoo in a beaker is very good)
lightly dampen the cloth (be careful here ... if you end up with water dripping down the screen ... you'll mess up the whole job)
lightly rub all over the screen ... remember they are made like a sandwitch - with a very thin membranes - if you press too hard ... you'll rupture the sandwitch
(the bigger the screen ... the less pressure it will take ... so do be careful)
if the screen is really bad ... repeat - don't be tempted to scrub - the surfaces are pretty soft and scratch easily.
dry the screen with another soft cloth to prevent streaking