How do you convert a European appliance to US voltage?

The voltage is probably not the biggest concern when installing a European appliance in the states as using an adapter or changing the cord should solve that issue. Find out the input voltage required then install a proper adapter to the plug or if its a larger appliance like a range or a dryer you'll just want to install a new power cord which can be found at Home Depot or Lowe's for fairly cheap. It may be a good idea for an electrician to install the cable or adapter to ensure its done correctly. If you wish to change the cord yourself just take a photo of the plug it needs to be plugged into and show a person at home depot and they will get you the correct cable.

What you should be concerned with is the Hertz (Hz). Hertz is the number of cycles of alternating current (AC) experiences per second. The United States runs off 60hz Europe runs off of 50hz. Now some devices can run off either if that is the case with your appliances your fine, however if the device is not build to take 60hz you'll end up damaging or ruining your appliance. google the manufacture and model number of the appliance and look at it's electrical specs for Hz and see what is listed. If you are unable to find the Hz you should email the manufacture or call customer service and ask them. I think that most major appliance manufactures build their appliances to run off of either, but you should check to be sure.