as mentioned the thick necked sweet onions only store 2-4 months while thin necked hotter onions store 6-12 months.
when you pull them leave them on the ground in the garden for 4-7 days, rain isn't an issue unless it rains the whole week! then move them to a shed, barn, garage, somewhere in the shade, out of the rain with some air circulation. i put them on racks i made from wood and chicken wire. just lay the onions on the racks using cinder blocks or saw horses to keep them off the ground. the bulbs can almost touch each other on the racks but i put the greens under the bulbs as i lay them on the racks. in 4-6 weeks they are cured. cut off the dried greens about 1" above the neck (NO water should be present, if there is use those 1st) and trim the roots to 1/2", store them in mesh bags at 37 degrees for best storage. initially in october i just put them into a 70 degree basement. as it cools i try to let in cold air at night or move them to a garage where it's cool. now they are in mesh bags in the garage where it's about 35 all the time. in a week or so i'll move them into the basement where it's 50-55.
it's that simple.
tom