The key here is remembering that the right-hand cup has a reverse thread, so it undoes backward. This is the same for all mountain bikes and most road bikes.
Before starting, check what kind of bottom bracket you have so you know the tool you need. Check the fitting. Almost everything is now a Shimano-type splined fitting with 20 narrow splines in each cup. ISIS bottom brackets use the same size spline, but the hole in the middle of the tool must be bigger to fit over the fatter axle. ISIS tools work fine on Shimano-type bottom brackets, though. Many of these bottom brackets have eight notches on the outside of the cup, and a few have only the notches. If your bottom bracket has both internal splines and notches around the outside of the bottom bracket, always use the internal splines to remove and refit – they make a more secure fitting and the tool is less likely to slip and damage your frame or the notches.
Step 1: Start on the left-hand side. Clean out all the splines on the bottom bracket cup so that the tool fits into them firmly. It’s worth getting a little screwdriver and picking the dirt out of all the splines before you start – they’re right down near the ground and tend to pick up all sorts of rubbish, which can stop the tool from engaging with the full depth of the slots.
Step 2: Insert the tool, firmly clamp on a large adjustable spanner and turn the tool anticlockwise to loosen the cup. It may be very firmly fitted. Take care not to let the tool slip off – it will damage the splines and it’s easy to hurt yourself. If the cup won’t come out easily, clamp the tool onto the bottom bracket. This calls for ingenuity; you need to fashion a washer of a size that allows you to bolt the tool on with your original crank bolt.
Step 3: Remove the left-hand cup completely. Check the condition of the splines and the threads, especially the cheaper plastic cups. These work fine and are light, but the splines can get damaged easily.
Step 4: Shift the tool onto the right-hand side and fit it firmly into the spline. Remove the tool by turning it clockwise. It can be tough to turn. Clamp on the tools if necessary. Bottom brackets often seize on, so a release agent like Shimano Get-a-Grip could be handy. If you use a release agent, make sure you’re in a well-ventilated place. Spray or drip on, and leave the release agent to do its chemical magic for half an hour before you try to shift the cup again.
Step 5: Remove the body of the bottom bracket. Don’t throw it away immediately – you will need to measure it to get the right size for your new one.
Step 6: Once both sides are out, take a look at the inside of the frame. Clean it out carefully. If it holds lots of debris, work out where the dirt is getting in and block up the hole. Make sure there are bolts in all the water bottle bosses, even if they have no cage mounted to them.
Source : BIKE MAINTENANCE TIPS, TRICKS & TECHNIQUES
See also bike maintenance tips, tricks and techniques “Internal bottom brackets : what size do you need”