The best way is to us MINIs special timing tool to set timing.  This tool aligns the pistons & cam so you can install the chain with correct timing.

Ken

better to have a mechanic come and look at it, never attempt to start the engine you might bend the rods..

That's what I tried to do the first time but it still wouldn't start. My timing chain had broke and so I have no clue where to set the pistons and all without the "reference position". 

With the right inner fender plastic out of the way, you should be able to mess with the cam chain while all else is still in the car. Of course, off comes the crank pulley, the cover, etc.  But all this stuff can come off in one hour flat

The cam sprocket has a mark ( a dot I think) and so too does the crankshaft sprocket, right?  Here's the key:  The cam chain has two link plates of a different color, for example the chain is dark steel and these two link plates are shiny.  They match up to the dots on the sprockets.  That's it.

You can be out by 3-4 teeth and not bend valves.  But it won't run unless it's right on the dots.  It will spit and pop and you'll wear out the starter planetary gears trying.  But once it's correct, vroom.  It starts and purrs.

I have a 2005 Mini Cooper S and the timing chain broke. I comepletely tore the engine down and now have everything fixed and back together, but my timing is off. I could really use some help figuring out how to set up the correct timing without being able to know where refernce position or anything like that is. It is severely driving me up the wall