I have had to deal with this several times recently as the people who I bought my house from didn't see fit to have a radiator in the kitchen, so it is often cold in the winter. There's 2 things you need to deal with, one is getting the butter up to temperature and the other is keeping it there. Radiators do work, but are often too hot and if you lose track you can end up melting your butter.
What I do to get the butter up to temperature is to cut the butter/shortening up into cubes and submerge it in warm water. Not hot water, but somewhere between normal room temperature and body temperature unless the butter is fridge temperature in which case I use body temperature water.
Once I make a buttercream I use a warm water bath to keep the buttercream up to a workable temperature, basically I have a roasting tray of warm water and set the bowl with the buttercream in it. If I'm piping it I will put the side of the piping bag in the warm water, which also works when piping stiff doughs like Viennese cookie dough.