Now, you can debate between leaving them plain or topping.
If topping, sprinkle your choice of toppings on each bagel (of course, you can top each one differently). I like a lot of seeds on my bagels, and find that order matters. Start with kosher salt, then the biggest thing you’re adding, progressing to the smallest. This makes sure the large seeds have a place to stick, then the smaller ones fit in the spaces in-between (like the parable about filling a jar). So my normal topping order is: Kosher salt, fennel seeds, flax seeds, sesame seeds, poppy seeds, and chia seeds.