how to fix a outside tap from bathroom through wall into outside wall?

You need to cut into cold water feed to sink tap, add a 'T' connector, connect pipe to sink into the straight ends and the outside pipe into the other opening after drilling through wall. Measurements need to be exact. Put pipe through wall inside a piece of hose-pipe to prevent damage to pipe. It will probably mean unfastening tap from sink unless you use compression fittings. Most DIY stores have leaflets you can see diagrams of how to.

Kew
June 2010