The easiest way to use strict mode is to use an IIFE (immediately Invoked Function Expression) like so:
(function()
{
'use strict';
var foo = 123;//works fine
bar = 345;//ReferenceError: bar is not defined
}());
To create a new-line in the console, use shift + enter, or write your code in a separate editor first, then copy-paste it to the console. Setting up a fiddle is all fine and dandy, but just test your code with the markup it was written for (ie: just clear the browser cache and test).
However, I’d urge you to install node.js, still. It’s a lot easier to test your code, or validate it (both syntactically and coding-style wise) using JSHint. There are also a lot of ways to examine and your code that run out of node.js, so it’s a really good development tool to have