I would recommend repairing the leak with the pump mounted on the engine if this is possible. Leaks often occur on the side cover, feed pump gasket, or on the shut-off lever. Most of these leaks can be repaired on-engine with small hands.
But if you really want to bring down the pump, do this.
1) open the side plug on the governor side.
2) turn the engine clockwise (engine rotation) until timing pointer is centered on the hole of the plug.
3) Engage the tool which Steve recommended. Tool will lock the pump camshaft. Engine crankshaft angle will be at approximately 14 to 16 degrees ATDC.
4) remove all low and high pressure lines and any electronic connection to the injection pump
5) Remove the vacuum pump in front of the pump. Loosen center bolt on pump camshaft clockwise. This is a reverse threaded bolt.
6) Remove flange mounting bolts of injection pump housing on timing chain cover.
7) Pull out the pump.
8) INSTALLATION: do steps 1 to 7 in reverse order.
Word of caution: This job may be difficult for a DIY.
I hope this helps.