welcome to the forum.
everything is correct so far. what you do need are the folds produced at step 10. these folds are the "zig-zag"-valley-folds of step 12. at step 10 hold the model at the point marked with the circle, for example with the left hand. then open the model with the two "corner-points" C and D. note, where these points are in the diagram of step 12. notice the zig-zag-line produced in step 10. these are all valley-folds. where this zig-zag-line meets the line AB, is another point that is not marked in the diagram - lets say E. you will have to change BE into a valley-fold in step 12, instead of a mountain-fold, EA will stay a mountain fold.
lets concentrate on C in step 12: change the existing valley fold into a mountain fold (only the part of the fold, until it meets the zig-zag-fold. make the same with D, change BE into a valley fold and you do have step 13.
you don´t have to sink anything. and don´t worry because of your english: someone wird helfen
