So on a basic level there are two things to consider when doing a reconsideration/appeal:-
(1) how do I meet the criteria for the benefit? As regards this, benefits and work do a 'PIP self-test' which gives a handy indication, and the points etc are accessible online (links below).
(2) what evidence can I get? the evidence needs to support the extent you meet the criteria for the benefit i.e. your limitations in washing/dressing/cooking/walking/etc. As CR states, a letter from your boss may have limited value depending on how well he knows about your difficulties with the activities.
Citizens advice do a good guide for requesting a reconsideration, and going to appeal which you may find useful (link below).
Hope this helps! Do let us know if you need anything further.