First, have their nails trimmed. I notice they scratch more often when they are long.
Second, get appropriate scratching surfaces for them to use. Get more that one and try different materials. The more places they CAN scratch, the less likely they will scratch where you prefer them not to.
Put those scratching posts in an area that they are likely to use them. Don't hide them somewhere out of the way as that will defeat your purpose.
Show them how much fun they are to use. Put catnip on them. Play around them so when they go for your hand, a toy or string, they grab the post. They will get the idea.