Ive done this recently - not hard you just need 2 buckets (or more) and a flexible head to your ratchet.
Get yourself 2x crush washers (number 16 here) http://bmwfans.info/parts/catalog/E3.../engine_block/
Make sure the water is cold before you do this.
1) Put the car on ramps
2) Open the cap to the coolant reservoir and put it in a safe place.
3) Remove the central and front belly pans
4) With a pair of pliers turn the blue radiator pulg 90 degrees and wiggle it out - about 2L will come out so have your bucket ready.
5) The coolant drain plugs are near the back of the engine and with your car at a slight angle it will help it all flow out
Mine had a dab of yellow/blue paint on them to identify them
Get your ratchet - sorry dont remember the nut size and attach your flexible extension and take it out - about 2L will come out each - fast - it goes EVERYWHERE as it splashes over the suspension.
On the UK drivers side there is a little heat shield to remove. Its very easy - its only held on with 3x bolts. Just watch your eyes as a lot of clag drops down from it when you remove it.
IfI poured fresh coolant into the resouvior it came straight out the drain bolts to confirm all was out.
6) Clean all the threads, replace the crush washers + plugs and refil with coolant. Run the engine with the coolant cap on and rev to 2.5k RPM 5 times (these are BMW's instructions). Turn off and check the level and top up.
I found that my level only dropped with a decent drive and needed 1/2 inch of distilled water.
Check you have no leaks (ahhhh my nightmares return!) and put the belly pans back on. You are then good for 3 years.