>>536795602
The way I go about it is moving the Corner Lips first, then the Outer lips second. Albeit that Hroth's face and/or lip structure is different, more often than not this trick can apply easily with every other race.
It just depends on the kinda of expression you want, but keep playing with them and find what suits you best. If it helps moving the Corner Bottom Lip above the Corner Upper Lip usually gives you a smile, so try it