I guess you need a diagnosis and appropriate treatment first.
Looking at the guidelines for the musculoskeletal section they look at the degree to which the symptoms (pain, joint stiffness, loss of range of movement etc) interfere with the persons ability to carry out activities, particularly walking distances, using public transport, climbing a few steps etc.
I guess they the review investigation results (x rays, bloods,scans etc) and doctors/physical therapist assessments to check that the severity of the disease fits with the severity of the symptoms that are contributing to the inability/difficulty carrying out everyday activities.
Also look for objective findings in physical exams eg "cannot straight leg lift" rather than "pt complains he cannot lift his leg"
This is probably where many claims fall down , plus,as pain is so subjective.
One person may have severe joint deformity and pain but has a higher pain threshold and so manages to function relatively normally.
Another may have less severe disease with a lower level of pain but find it difficult to function as their pain threshold is lower.
I guess I would be making sure that I had difficulty walking etc, and get a doctor who specialises in musculoskeletal problems and is familiar with the process of applying for disability.
If not then I would be taking this
1.00 Musculoskeletal System-Adult
with me to the Doctor and saying "this is what we need you to do"!
Also document periods of missed work and reasons as they look at the presence of symptoms/limitations that are intermittent over a period of time.