Scoop the royal icing or softened paste into the piping bag. Don't fill the bag completely, otherwise it will be impossible to close it. Fill it to about half way.
If you want to use a piping nozzle, cut off the tip of the bag (see the photo below) and put the piping nozzle in the bag before you scoop the royal icing into it. Don't cut off too much because the piping nozzle will fall out.