How to convert a petrol engine to diesel

how do you change a mrk2 golf engine to a 1.9 mrk3 diesel engine

It can not be done you will have to buy a 1.9 mrk diesel engine...
Reason for this is that petroleum engines and diesel engines are designed differently and the fuels have different requirements for mixtures, cooling, combustion temps and cylinder size which make them incompatible.

it could be done but as (thetechempire) said it does require alot with the anmount of money spend to do this you are just aswell buying a 1.9 mrk diesel engine

You could buy 5 new diesel engines for what it would cost you to convert a gasoline engine and the converted engine still wouldn't run right. Some things just aren't meant to be. :-)