Idiot guid needed confused! The whole coolant thing.

Here goes i've searched and read so many versions i'm lost.
My coolant reservoir looks low should it be filled to and stay level ??
because if its worryingly low. Does that mean the rad is also empty and in this heat now it cant be good can it. so a idiot guide to understanding the system would be nice

(I bow to your knowledge) its like all things once you have done it once it becomes easy cheers

possibly on a 300 tdi you have a bung on the top lhs of rad and one on thermostat housing ,fill with coolant water mix 50%ratio (antifreeze)till holes over flow then refit plugs ,header tank will be then overfull but it doesnt matter as level will sort it self out any excess will come out of cap harmlessly , after drive check rad is hot allover proving its cooling efficently if not replace ,top hose getting hot proves thermostat is opening ,check level each morning till your confident its stable

thanks all set up ready to be sorted