Hi,
A little background, im Dutch and in my language there is no TH sound like the one in English (as in 'three or thief', and at the end of a word, as in 'with, month or death, etc') and it's giving me some difficulties.
When I pronunciate three for example, it sounds more like "fhree", but when I hear it on TV or somewhere else I can hear a slightly harder T sound in it, which I'm unable to reproduce.
The method as I learned it is to put your tongue between your teeth against the upper teeth and force the air through the gap.
Could someone explain the

exact method on how to make this sound in detail?
The same TH + plural sound is difficult for me as well. Like "months" ends up sounding like mon-f-s.
How am I supposed to pronunciate this properly?
Thanks for any help in advance.