As I said before it depends whether you need access to the roofspace at 90 degrees to the main roof space.
call the main roof 1 and the roof at 90 degrees roof 2
If you don't need access then thats the easiest cut the main roof as normal then set up your ridge for your other roof that runs into it and cut the common rafters on that.
You then need to set up a lay board which you nail to the common rafters on the roof 1, down from where the rigde of roof 2 starts to the bottom of the valley.You then need to cut jack rafters which sit on the lay board and run upto ridge 2.
If you need access into roofspace 2 then its more complicated as you need to miss out the common rafters on roof 1 for the width of roof 2.
Set up your ridge and common rafters on roof 2 you may need to trim between the common rafters on roof 1 if the ridge is below that on roof 1.
Then you need to run a valley blade (usually deeper timber than the common rafters)from where ridge 2 meets roof 1 down to where the bottom of the valleyhits the common rafters on roof1.
You then need to run jacks upto both ridge 1 and ridge 2.
If you can understand it from that then your a better man then me and I wrote it
If you don't want to buy a book on roof framing then at least get this it will show you how and gives you all the angles cuts and lengths for any pitched roof it has afew drwaings too and that may be all you need.
http://www.amazon.co.uk/exec/obidos/ASIN/0632057653/026-5624704-7179661