Cutting/shortening a steertube is pretty straight forward. all you need is a hacksaw or pipecutter (depending on what you may already have or prefer) and to know how much shorter you want the tube. Make sure you cut as straight as possibe, and remember to press the starnut in deeper before you cut.
If you want to go from threadless to threaded then you have to cut down your steerer tube to a certain length (not sure what that length is) then you will have to get the fork threaded
If you are going from threaded to threadless it probably isnt possible with your current fork because after you cut off the threaded part the steerer tube will most likely be too short to connect the stem to