? are you trying to disassemble? Which mini-mill? Did you break a gear? If it is the commonly-broken gear on the mini-mill, you won't need to remove the spindle. The spindle shaft has two diameters, and all the gears and spacers are a bugger to reinstall. I really would not disassemble the spindle unless you are absolutely certain you need to.
If it is Harbor Freight, look at the manual. The exploded view is one of the last pages, and also availalbe as PDF on littlemachineshop.com
The spindle is pressed into place in-between two bearings. I had to replace the bearings on mine this summer. NOT fun.
The gear box needs to come off of the mill to do it properly. Meaning removing the motor, controller, and the micro-feed adjustment. I would not even bother unless you take the gear box off of the mill.
When the gear box (and spindle!) is out, set it in the press to remove it, or on a vise if you don't have one. You can remove the spindle by hammering on a block of wookd to protect the end of the spindle. I would not hammer on it directly because if you mushroom it, it may not hold collets correctly.
The bearing on the end will push out because the spindle pushes it out. There are keyway keys to keep track of, too. Those can get lost real fast.