How can I place treads on these stairs around a corner?
So you have two issues: the cut angle and the gap.

Cut Angle

Whenever two boards meet at and angle, you have to cut each board by half of the angle in order to get the edges to line up.  So, in the top-down picture with the pencil, lets say that is a 30 degree angle.  Cut each board at 15 degrees.  You can't cut the board to line up with the stringer - you have to "split the angle".

Mind the Gap

Again, using the top-down picture, the board on the right will be in the "correct" position because its naturally perpendicular to the stringers.  The left board hits the stringer at a 30 degree angle, so either you will have to cut a small, triangular notch out of the board, or make a notch in the stringer for the board to fit into.  Notching the board (probably with a jigsaw) will be the easiest option, and when you're done, the fronts of the two boards should line up.  To make getting the notch in the right place easier, cut the notches with extra board hanging out over the edge before trying to cut the 15 degree miter to meet the other board.

I may be able to add a drawing later if this isn't clear enough, but I think it should get you going in the right direction.

*Obviously I made up the 30 degrees measurement.  Get an angle guide to measure each one before cutting.