No stupid question at all
How: Dear saint xy, pray for me… Basically, it is the formula of the Ave : Sancta Maria, mater dei, ora pro nobis peccatoribus…
It is not exactly the same way as ypu pray to God, because it is ONLY God who can grant prayers, but the saints may help you in praying - like ypur parish might pray for you if you’d have an acciddent or alike. They are members of the communion of saints, living and dead, all praying together
What intentions: Every single one!
Does it matter which saint? Hm, it matters for the purpose of personal identification witht he prayer and the saint. For example, Maria is called on quite frequently, no matter what intention the prayer has, but many saints had experiences during their live that makes them easy to identify with for some prayer intentions (then their patron saints), for example if a saint suffered very much and was delivered miraculously, he might become patron saint of hopeless cases, or if he was very found of studying,t he patron saint of students/teachers.