This is the simplest of all the bonuses. All you need to do is register a real players account at the casino offering this bonus, and you will receive the money or free spins. You do not have to make a deposit to receive it. Sometimes it will be automatically credited to your account and sometimes you will need to claim the bonus via email or a submission form on their website. But essentially it is very simple.
There may be limitations on this bonus at the different casinos, so you must make sure you read their terms and conditions and make sure you understand the rules. One of the limitations may include things like a maximum amount you may withdraw using this bonus. Also, some casinos may not have this cap, but would require you to make a token deposit before withdrawing your winnings.
Please also note that no deposit bonuses mostly also require a different playthrough than a normal signup bonus.