Every one star = 10 points, therefore you need a total of 100 points for a 10-star ranking. There are 102 points available in the game so you get two freebies

Difficulty:
Hard=20/20 points
Normal=10/20 points
Easy=0/20 points
You must play on hard for 10 stars.

Play time:
Under 2 hours=20/20 points
At 2:00:00 you lose a point and every additional half hour you lose another point.

Saves:
0 saves=12/12 points
Every save you make you lose a point.

Continues:
0 continues=10/10 points
Every continue you make you lose a point.

Enemies killed:
120 enemies killed=20/20 points
You get one point for every 6 enemies killed.

Scrapbook Notes:
52 notes=20 points
You get one point for around every 2.6 notes you get.

That's all the requirements. It is very easy to get a 10-star ranking in this game, unlike the other Silent Hills. Just be sure you know all the areas well.