Unfaced trims are usually hand sewn. Are the pillows already made and you are trying to add embellishment to them with the trim? If so, you will need to deconstruct the pillow's 4 side seams in order to correctly use the facing surface of the trim. Remove the pillows case (outter covering) so you are working with flat surfaces, rip out the 4 seams, place the trim between the ripped seams (right sides together!) and resew with the trims facing now being a part of the resewn seams. Turn right side out... and voila your pillow casing now has the trim attached.
What has been will be again, what has been done will be done again, there is nothing new under the sun.