take part of the 1ng/ml and add an equal volume of diluent to make 0.5ng/ml.

take part of the 0.5ng/ml and add an equal volume of diluent to make 0.25ng/ml.

take part of the 0.25ng/ml and add an equal volume of diluent to make 0.125ng/ml.

etc.

So, 1 ng/ml is my stock and if it comes in 1ml then:

I take 0.5 ml of my stock + 0.5 of H2O -- 1

0.5 ml of 1 + 0.5 ml of H2O -- 2

0.5 ml of 2 + 0.5 ml of H20 -- 3

and so on

Am i correct?

if your stock is 1ng/ml and you then take 0,5ml of this and add 0,5ml water you will have in total 0.5ng (since you took 0.5ml from a 1ng/1ml stock) of the stock in 1ml, thus 0.5ng/ml.

And you can keep doing this:

take 0,5ml from the "new" stock , add 0.5ml water... you keep diluting it 1/2 each step.

But this is pretty basic, you should understand this, know how this works.