I don't know if you want to do these ITH or by hand, sew since I do them by hand: what I did was trace the metal hair clip on to cardboard ( I used a cereal box)I added a bit more all the way around maybe 1/4", then I use this as my pattern this is step1,step2,cut it out,step3 layer 2cd pc on top of first & cut it out, next row on right pics step4 bad pic but you can just make out a bit of a white dot this is a T cut @ the top of the pc that will become the backing. step5, shows the 2 pcs stitched together ( I used white thread hoping that it would show up better, but normally I would use same color as felt)this shows how to slip the clip in the back slit.step 6 shows the slit covering the top of the back. I use fabric glue & just add a dot to the clip @ the back & squeeze the felt T (cut T) together. All you have to now is sew out a clippie design for the top I hope that this is helpful
Instructions how to make felt hair clips.
If you are a member of Sew Forum and can do embroidery designs there is a free design there for the hair clip covers.
I think these are a great idea. Follow the instructions then you could sew or glue the hearts on to the clip covers.
I think sewing would be better. The glue might crack with the clip being snapped back and forth.
I am sure you can do these with a regular sewing machine but there is an ITH design specifically for them..here's a link..