Proper way to hang valance over kitchen window between cabinets
Valances usually sit on a couple of beefy L-brackets that are double screwed into the wall (preferably at wall studs or window header). Once the L-brackets are solid you literally just slide the valance with installed curtain or drape assembly onto them to then install a single screw through the top of the 'L'... so the valance can't slide off.

As far as width of the valance goes, it's up to you. I prefer the curtains/drapes to dictate the width and have them withdraw into their own pockets to pull back to the trim (maybe 2-inches in your case, depending upon hardware stacking). So, you don't have to fight them for tilt-in glass cleaning.