Enter config mode, create a local administrative user (in the event of RADIUS failure), and enable AAA new model:

config t
username admin privilege 15 password 0 <ADMIN PASSWORD>
aaa new-model

Specify the RADIUS server information:

radius-server host <RADIUS SERVER IP> auth-port <RADIUS AUTH UDP PORT #> acct-port <RADIUS ACCT UDP PORT #>
radius-server retransmit 1
radius-server timeout 1
radius-server key <SHARED SECRET>

Configure AAA options:

aaa authentication login default group radius local
aaa authentication login localauth local
aaa authentication enable default enable
aaa authentication ppp default if-needed group radius local
aaa authorization exec default group radius local
aaa authorization network default group radius local
aaa accounting delay-start
aaa accounting exec default start-stop group radius
aaa accounting network default start-stop group radius

Write configuration to memory:

wri mem