Water Coolant addition

[tablahan]

Okay guys i know this is in the manual but hear me out.

I've noticed that my water coolant level is low when i looked at the reservoir's (the plastic tank) level. So o presume that this has to be topped off.

Can i just top this off by adding coolant?
If yes what level should i follow as guide?
What conditions? engine hot, warmed up cold... etc?

The reason i need to ask this is i have a BMW 523i and when i added some coolant i screwed up the cooling system because of the way i put it in (according to BMW dealer who serviced it) and they (BMW dealer) told me that they should be the ones to put in the coolant if it needs topping off.