I usually pronounce it longways as if it's not an an acronym at all, but in a kind of breathless way:

omg - oh my god!
lmao - laughing my ass off
smh - so much hate (I know that's not what it actually means)
same for ikr, idgi, jfc, iirc, ayrt, tbh, lbr, wtf, etc. etc.

OR I pronounce it as it looks:

lol - lawl
orz - oars

idk is the only one I can think of that I individually say all the letters, i-d-k