How do I climb the roof after killing the Big Boo?

This YouTube video shows the way. He takes shortcuts by sliding down parts of the roof, but it should be clear how you can do it without the sliding if that's too difficult (see DMA57361's answer if not).

A quick glance at a few game guides shows they all seem to give the following approximate advice:

After killing the Big Boo check left and right and you should see small flat areas above the windows, long jump to them. From here either crawl up the slippery slope towards the centre of the roof or long jump over it (crawling is probably safer). Then follow the flat path here to the star.