To join the police force you need to fill in an application form supplied to you by your choice of force. The stages are carousel (a test of how you would deal with different situations), medical examination, physical assessment and interview (not necessarily in that order).
The police will carry out background checks and request references and if all these are satisfactory and you have successfully completed all the stages then you will be accepted as police constable.
Each police force recruits individually and for details on any recruitment drive you need to contact the force you are interested in joining.
For further advice on how to become a police constable see related website .