Mostly by getting the thetan in communication with the body.
The Touch Assist has a patient close their eyes and the practitioner say "feel my finger" and touch various parts of the body, until the patient experiences some relief.
There are other exercises, the one for fever involves having the patient hold things still.
The big thing is an underlying assumption that one is only sick/accident prone because they are connected to a 'Suppressive Person' - an 'SP' and are therefore a 'Potential Trouble (for the organization) Source, they're said to be 'PTS'.
There is a whole course about handling PTS/SP situations. It involves lots of different techniques for aiding the PTS person in identifying their SP or SP's and disconnecting from them.
SP's are said to only make up 2-1/2% of the population, but through the people under their thrall have a disproportionately large impact on society. Amongst other features their the ones surrounded by the sick and dying, chaos and confusion, yet they seem to thrive. (Think clintons)
Curiously, SP's are almost never found within the Church, but are frequently those people outside the church who are part of the client's non-scientology support network...