I don't know if a tutorial exists but it's really easy.
First of all, you need to have added the attraction you want as a step - you can't just add it as a FP.
Then you go to the FP+ Selections box and add the FPs you either want or already have. Make sure the time is right and you haven't put pm instead of am or vice versa. Hit the Save FP+ Changes button.
Then optimise. You should then see that the plan uses your FPs. Sometimes it won't because the plan doesn't think it's a good choice. You can either pick something else in that case, or go to Advanced Options in the FP selections box, and tick the 'Force optimise to use all your FP+ reservations' check box.
Whichever you do, optimise again.
Hope that helps.