5 Answers

If you have never done this before it's better to get the professionals in, other than that there are some tips here.

http://www.wikihow.com/Install-Wood-Stairs

Some tips here

http://www.servicemagic.co.uk/resources/staircases-for-loft-conversions-design-tips-ideas/#.UI2M9sVG8uE

There is a lot of art and science required to figure the proper rise and treads for a set of stair stringers.  Skilled craftsmen often have difficulties, so that is why others are coaching you to hire a professional.  Once you have done it once or twice, you will see what I mean.  You must have excellent math skills, and carpentry skills.  Whatever you do, do the finish cutting by hand, as using the circular saw for completion of each cut will give some overcuts, which actually will weaken the stringers.  So stop cutting each 90 degree cut before you reach the intersection of the previous cut (comprising the 90 degrees).  Then with a jig saw, or a hand saw, complete each cut to connect the perpendicular cuts making the right angle of 90 degrees!  Another tip, do not skimp on the lumber for these stringers, use 2 in. by 12 in. planks, anything less will not be proper, stable, and does not meet codes!  Take time, figure on paper, and double check before rushing into cutting the stringers, it is expensive to make these!  Try a small set for a deck stairway, of lets say 3 stairs, to learn how first!  Good luck!!  (you will need it!)

As Romos says , if you havn't done anything like his before get a professional in.

this is not what you begin with learning carpentry, it may look easy but it harder than you think, there a whole course on making staircase. different  height ,width, and can wast a lot of material ,if your not in for the challenge?