how do I assign static ip address on time capsule

I want to assign static ip addresses to certain devices in my home but not sure how to do it through the iMac and Air Port/time capsule

The AirPort Time Capsule will need to be set up and configured to be the "main" router on your network. That would mean that it must be providing DHCP and NAT services to all of the devices on your network. Another way to say the same thing would be to state that the Time Capsule cannot be set up in Bridge Mode.

Is the Time Capsule set up as the DHCP and NAT router for your network at the present time?

Not sure as I don't own an Apple router but on mosdt modern routers there is a way to set, that is SET, a constant IP for devices by their MAC address, that is Mac, Media Access Control address not Mac computer.

I would think Apple has included that feature in their Airport routers.

use DHCP Reservations under Network Tab to do that...