Without actually seeing the designs ...
They already have heat-n-bond or Wonder-Under on the back of the design, right? ... assuming you do, just trim the design about 1/16" OUTSIDE the image (this is what will be covered by your applique/satin stitch) ... remove the paper, place it on the garment where you want it, move it around to see what you can and where ... IF you have to cut any off, gently tap the side you are NOT about to trim away (so you don't lose your placement), trim as needed to get it to fit right ... then finish pressing, satin stitch, press to smooth, done.

IF it were me doing this printed applique = I would ignore the background color "around" the edges of the design, since you won't see it after it's stitched, anyways ... I would use the same color of the "outline" of the image = like for instance if the drawing/image had a black outline, I'd use black ... that way it'll blend in a tad & help hide imperfections. .......... you COULD use the same color as the fabric you are attaching it to, but if there's a black outline to the image = the contrast of your stitches against that black will show imperfections in the stitching more, than trying to blend it in ...
That's how I did design ...
+ you could TRY a bit of each & then decide. That might be best, if you're really unsure going into it

! ... use the scraps from the part you cut off = so you don't have to rip anything out of your garment

!
good luck!