Sit on the edge of the bed, pull trousers over feet and legs, make sure feet are clear of trousers. When you have the waist band up to upper thigh level, slowly stand up and pull trousers over your fat arse. Fasten trousers at waist. Remember to do up the fly.
I cannot be responsible for your lack of co-ordination but the above described method is the one I have seen employed with success by many ELDERLY men in hospital wards over recent years ~ they too have to be reminded to zip up their fly, usually by a nurse.
