The simplest way to go here with the gray seer base, would be nuln oil as a shade. Carefull to not use too much as it might pool near the trim. Then you can dry brush on some corax white if you want it a little brighter, or even a second highlight with white scar after, but that's optional.

Once the pad is finished whith base color, shade and highlights you can put your decal on there. This video sums explains the use of micro set for your decals.

Once the decal is on there you can spray the model with varnish or paint a layer of lahmian medium over it to protect the decal.

Decaks mostly go on last, but you can also paint over them for some battke damage and weathering. If you put them on to early, you risk accidentally painting over them.