1. You must be kind of an idiot to try to ski behind a horse.
2. You will probably hurt yourself (maybe even badly) doing this and that is not my fault. It is your fault (see number one ).
3. You really ought to wear a helmet (see number 2). Even though you will probably still get hurt, you are likely to survive if you wear a sturdy helmet.
4. In addition to your helmet, you'll need:
-one competent rider
-one idiot (you)
-Skis (or a snowboard)
-A watersports tow handle (like you would use for waterskiing, wakeboarding, etc.)
-(optional) one ridiculous costume
-(optional) cones, ski gates, jumps, or other additional challenges
-(optional) one ambulance
The rest is fairly self-explanatory. Attach the rope tow to the saddle horn and give yourself a few generous yards of rope between you and the horse so that you don't get in each other's way.
Let 'er rip!