An - the

Nach - not

Ar an - on the

Án - our

You drop the sound of the original first letter and replace it with the sound of the new letter. 

ex. bia (bee-ya)

an mbia (an mee-ya)

"nG" is most easily pronounced by attaching the "ng" (as in "sung) sound to the previous word. 

ex. án ngeata

(awng A-tuh)