You can find the tool via your AdWords campaign management page under the Shared Library section. Simply name your budget, assign campaigns you want included and set your daily budget amount.
While this is a good option for advertisers who do not have the time to manage their campaigns and adjust budgets frequently, I would not recommend it to more savvy advertisers with multiple campaigns, who are watching conversions closely. Your campaigns are likely converting very differently, and I recommend adjusting budgets based on conversion data, rather than traffic, which is what shared budgets will accomplish for you.
One creative way you could use shared budgets is to assign your top performing campaigns (that you already know tend to convert better for you) a higher percentage of your total available marketing dollars. For example, if you have a total budget of $100 per day, you could create a shared budget of $70 for your top converting campaigns, and a separate budget of $30 for your low performers.