encoding response value to base64 and using it on another test

Im trying to do some testing using jmeter but im facing an issue trying to do some complex stuff.

I have a "login" http request test that comes back with a response which includes an auth_token. I need to add ":" at the end and encode it to base64 to use that value on the request of another tests. Ive been reading that it can be done using beanshell but I could not achieve it yet. I will appreciate if someone could give me some steps to perform this task.

Thanks