In each side of the stair we gonna use two large and strong beams to attach the steps.... so we need to measure it and be sure that fits..
In this case we use a 10' beam and 2" thick.
We have to draw a scheme for this to make sure we are in the right direction.
First you need to se where you wanna put your stair... we use a very easy and secure way to attach this beams to the floor and to our second floor. (see the image). We use the first beam of the second floor to lay our stair... and we only put 4 guides to make it go straight, not for support part of the weight.
In the down part we put two strong steel parts to support the weight and this two parts goes attached to the floor by two big 1/2" screws. This is this enough to keep the stair in the floor.