Hello, I hope the following is of assistance.
When replacing the fuel pump before removal you need to have the engine set at 50 degrees BTDC and insert a timing pin into the crankshaft position sensor hole, this pin slots into the flywheel. When fitting the new pump you then leave the timing pin in the flywheel and fit a timing pin into the hole in the fuel pump sprocket that pushes through into the hole in the fuel pump. The 2 diagrams below will assist with this.
Once the bolts are tightened remove the timing pins and turn the engine 2 turns by hand and refit the timing pins to the pump and flywheel. You then undo the 4 pump sprocket bolts just enough so the sprocket can move and turn the sprocket anti clockwise with a suitable tool, this will tighten the timing chain between the crank and pump and while holding the tension retighten the pump bolts. Below shows a diagram to assist.
When in the correct place the timing pins should fit in the flywheel and fuel pump and you should be able to access the 3 pump retaining bolts through the holes in the pump pulley.
If you didn't time the engine before removing the pump then it will be a little awkward as you will need to locate the correct slot in the flywheel for the timing tool to fit in, 1 slot in the flywheel is deeper than the rest but it is difficult to see, you will probably need a mirror and light to inspect the flywheel well enough to find the slot.
I hope this helps but if I can assist further or if you have more questions regarding this let me know and I'll reply as soon as I'm able to.