WHAT YOU NEED TO KNOW
Setup feels loose: It should feel absolutely solid. A millimeter's play around the axle may indicate the bearings are worn and need to be replaced.
Every 3-5,000 miles/3-5 years: Approximately. This part of your bike receives some abuse from you and the elements, and water builds-up inside the frame so it may also be prone to corrosion.
Drain the frame: Draining the frame every year helps, but sooner or later you need to take it apart, clean, lubricate and put it back together again.
Shift chain off chainrings:: Shift your chain onto the smallest front chainring, and from there off the rings completely until your chain rests on the frame. Mind the paintwork!
Remove crank on 'drive'/right-hand side. (The 'drive-side' is the one with the chainrings).
Insert crank extractor: Unscrew the dust cap and retaining bolt at the centre of the crank arm. Then fully extend the crank puller by drawing back the inner driver from within the outer sleeve. Gently screw the tool into the crank until it stops.
Remove the crank on the left-hand side:
Repeat the process described above for the right-hand side.
Lie the bike on its side ready to remove the bottom bracket.
Bottom brackets have two sides - a cartridge side, which gives access to the bearings unit, and a lockring side, which keeps the bracket in place. The lockring could be on the left, or right hand side - it depends on the manufacturer. Either way, remove the lockring-side first.
Unscrew lockring: Insert the bottom bracket tool into the splines of the lockring, grip it with a spanner and then unscrew the lockring.
On new Shimano bikes the lockring is on the left, non-drive side, and you unscrew the lockring anti-clockwise (the usual way for unscrewing). This may need some force. If it sticks hard, ask someone to tap the bottom bracket tool with a mallet as you attempt to turn it.
Grease and identify left- and right-hand sides: Clean and grease the thread inside the frame, on the new bottom bracket sealed unit and the lockring. Check your new unit is compatible. Identify the cartrdige and lockring sides - the cartridge itself may be labelled L (Left) and R (Right) to assist.
Check that cranks and spindles are free from grit and grease, and then slide the first crank onto the spindle. Grease the sides before you slide it on. Finally, screw in and tighten the crank bolt. Then repeat for the crank on the other side, after checking its alignment - opposite cranks point in opposite directions.
Check new setup is absolutely firm: There should be no play around the axle, and the cranks should not wobble, or feel loose when they rotate.
Test ride: Go for a short test ride on a short road, taking a few tools with you. If you feel anything strange, stop and check nothing is working lose (especially the crank bolts).