What model? I just pulled the pump on my 89' 325i, it was really very simple:
89' 325i has one pump located in fuel tank:
1. Lift off rear seat, on passenger side there is oblong plate. Remove plate to find top of pump assembly.
2. Disconnect hoses and harnesses, make sure that you clean up all of the dust!
3. Once hoses are removed, you will see 4 small nuts on top of pump assembly. This is the fuel level float, you'll want to pull those nuts off and pull it out(it comes straight out, no problems).
3. Twist pump assembly counter clockwise about 1/8th. Keep in mind that there is an o-ring between the assembly and tank(don't lose it, you may need it again). You have to wiggle and manipulate the assembly to get it out, it's a pretty tight fit! Keep in mind that there is an oblong screen at the bottom of the assembly, if you're not careful knock it off and be fishin' in gas(I speak from experience!)
4. The pump is actually attached to the assembly, so you'll have to dissconnect the wires from the old pump and solder them onto the new one.
5. Reverse 1-3, and BOOOYAHHH, you've got a fresh pump.