I just took some measurements of both the actual decal (not the surrounding carrier material) and the area where I intended to apply it - subtract the size of the decal from that of the location and divide by 2, this is how far from the edge you need to place the decal. Do this for both vertical and horizontal positioning. Once the edge position is found I just laid some blue painters tape at that point, peeled the facing off the decal and set it against the tape guides, did the squegee thing, peeled the backer off, then removed the tape.

Measure twice, stick one.