I don't know about the P91, but I have a Ruger P95 that fieldstrips into 5 parts fairly easily. I assume the P91 would do the same. I am going from memory here so the following may have little to do with reality.
Eject the magazine and lock the slide open. Push the extractor (I think?) forward and down from where it sits in the rear of the magazine well (reach into the top of the gun with your finger). It should pivot forward and down. Then release the slide.
On the left side of the gun there should be a small line cut into the slide where it meets the frame. There should be another line cut into the frame itself. Pull the slide back until the line in the slide is lined up with the line in the frame and then push on the slide stop where it sticks out from the right side of the gun. You should then be able to pull the slide stop the rest of the way out from the left side of the gun, while keeping the lines in the slide and frame lined up. Release the slide and push it forward off the frame. Turn the slide over and remove the spring components and barrel from the slide.
If this doesn't work I am sure Ruger would send you a manual if you asked them.
Hope this helps,
DAT