Double the amount of work for the specialization-focused muscles.
Decrease the volume for everything else so that the overall weekly volume remains the same.
Spread the volume for the specialized muscle(s) over 3 non-consecutive days.
A specialization phase should last 2-4 weeks.
After the specialization phase, reduce the volume for the targeted muscles to 1/3 of your NORMAL volume (NOT the volume during the specialization phase).
Although not 100% necessary, I like to increase calorie and protein intake on the days where you hit the specialization-focused muscles.