Well, did the instructor say on which part of the body or what the focus was supposed to be on? Usually in a focused assessment you zoom in on what the problem area is. Have you been shown how to do a head to toe assessment yet? If you have then you have the basis down. Just take an area of a body and perform an assessment based on what was taught to you for the head to toe exam in that area of the body.
Let's say it is a focused assessment for the foot area; you would check ROM, color of skin, capillary refill time of the toes, condition of skin, any gross abnormalities (open area, deformity), condition of nails, sensation to the foot, any pain (if so does it radiate, the rating, the quality, length of time), any previous history of feet issues, can the person tolerate weight bearing. This is some of the things I can think of off hand, but it is at least an idea.
Sorry if this confused you, I hope it helps a little.