The basics (importing the stem files, panning them left and right, setting their relative volume levels) are very easy to achieve with Audacity, a well-known and free piece of software.  It can also apply a wide range of effects - reverb, compression, EQ and more; but having the software doesn't turn you into an accomplished audio engineer overnight, so prepare for lots of trial and error – and keep saving your project!

You can also do this to a limited degree in the Jam Session area of bobdylan.com's Studio A Revisited Micro-site, though this just  lets you listen to a mono mix containing your own balance of the four stems - and you can't save anything.  It offers this same facility for the master take of "Like A Rolling Stone".