Yep it`s kinda tricky to put it all together if it`s your first time , well first things first, wash everything with diesel or gasoline or any solvent you have available in your area, after thoroughly cleaning it, wipe everything with dry cloth or flush it with air blow gun if you are using diesel, after all parts are all dried and clean put multipurpose grease on the ball screw casing, in that way you can stick all the ball bearings inside piece by piece using a small magnetic screw driver, after all the ball bearings are stuck inside the ball screw casing gently screw back the threaded rod all the way until it reaches thru the other side, you may have to pry back some of the balls when it loosens in the path way of the thread, patience is the key on working ball screws, the first time I overhaul ball screws it took me thirty minutes to figure out how to put it back haha, but now that im familiar it`ll take only 2 minutes for me to put it back first time = hard, frequent = boring hehe
Edit: sorry I forgot, you need to wash again the ball screw with diesel to liquify the grease you put on earlier, to be able to make the ball screw as smooth as possible when it rotates, on our units I use cooking oil for lubricating the ball screws after it`s been overhauled. I use thin oil on it to be able to flush the dust debris easily with air blow gun while cleaning it after an 8 hour work, the grease tends to hold the dust debris firmly and will get stuck again in no time because it`s hard to flush it because it`s too thick.