There's nothing all the difficult here. In fact, changing the air filter is probably the easiest maintenance act you can do on the Shadow. It's also one of the most important you can do on a regular basis, second only to changing the oil. How often to change the filter? I recommend every 5000 miles or every 6 months. If you live in an excessively dry or dusty region, I would change it every 3000-4000 miles or every 4 months. Governed only by expense, it's impossible to change your air filter too often.

First, remove the six air filter cover bolts using a 4mm hex wrench. I suggest using a magnetic parts tray to hold the bolts. After you remove the bolts, carefully remove the air filter cover. Be aware, this thing (the stock one anyway) is simply chromed plastic. It will break easily and scratch even easier. Place it on a towel while you replace the filter.

And there it is. Simply pop it off and replace with the new one. Personally, I use K&N's filters (model HA-7500 fits the Shadow A.C.E. 750) as they don't need to be replaced, only cleaned and recharged (oiled). Whatever you choose, once the filter is installed, carefully reinstall the cover and you're done.