Literally just finished (2 hours ago) replacing the fuel filter on my 2001 1150GS fitted with an Adventure tank.
Straight forward job made easy with a bit of preparation, took me 30 minutes start to finish (done it before though). Buy the two new O'rings (access plate on side of tank, other for filler cap) in advance as the old ones will have expanded & will be too big to reuse immediately. Keep the old ones as spares as they'll shrink in time and can be re-used.
External filters are easy enough although I've not bothered. Some like to attach them to the bike near the r/h hand throttle body - I think this is a little exposed. If you've got a non-ABS bike, best place to locate the filter is beneath the tank, wrapped in some foam in the space where the ABS pumps would normally be.
If you remove the access plate, you'll see that there's a 2-3" straight length of fuel pipe at one end of the fuel filter & a 4-5" length at the other end (180 degree bend). Remove both & replace with a 10-12" length of fuel pipe suitable for high pressure injection systems. Worth buying a handful of 12mm Jubilee / Hose clips when buying the fuel pipe as you'll need a couple to replace the crimped on BMW clamps at either end of the new fuel pipe.
You'll need a second length of fuel pipe, attached at one end to the outlet spigot on the access plate and leading to the quick fuel disconnects (if not fitted, now's the time to do so). Length of this fuel is determined by your choice of location.
If fitting quick fuel disconnects, have them pointing in opposite directions to prevent them being connected the wrong way round:
Whenever you remove the tank, always grease the O'rings on the quick fuel disconnects to prevent them perishing & causing fuel leaks. I carry a spare connector & a few O'rings.
Wait till you're low on fuel i.e. low fuel light on before starting.
I've heard of the 5mm access plate studs snapping. Grease the threads on refitting, tighten sequentially & don't overtighten them.
If using an Adventure tank, now's a good time to fit another right angled quick fuel disconnect to the left hand side of the crossover pipe, saves pulling it through on removal & pushing it back over the engine when re-fitting the tank.