Segregate 2 LANs 1 WAN gateway

Have to create two seperate LANs(named Big with 18 devices& Tiny with 3) with no traffic crossing from one to the other.
WAN connection is a Cisco IAD/Router belonging to the ISP which I’m (of course) locked out of.
I want to connect the Big LAN via a 24-port Switch directly to the Cisco using the default scheme 10.0.1.1(Gateway), while the Tiny LAN connects via a router to a single port on the 24p-switch and should use 192.168.1.254(Gateway)
The switch is a Dell PowerConnect 3524 w/VLAN capabilities.
The Shared WAN port could/should be Port 1 and Tiny LAN router is connected to port 24 (this thing also has 2 stacking ports)
The more I try to configure it the more worse off seem to get and I have till Friday to get it right.