You have several ways, although the easiest is to pay for them:
Buy or activate the Diamond Card so you are going to be able to collect diamonds daily. Obviously, the Diamond Card has a cost, but it is the easiest method.
They are not very common, usually, expire quickly, and they generally work for certain geographical areas. But of course if you find them, they are great and it’s the most realistic way to get free diamonds. In case you want one that works from the date of publication of this article: JXRPL611BZVU87XP.
Recommendable: Although it is difficult to find them and make them work.
Apps to earn money
We do not recommend them at all, if they really pay, they take a lot of time for it. For example, there is one that for each action they give you 20 coins, but you can not exchange them for money until you get 500,000 coins. And there are others that seem to pay well when you download a game, but then, they ask you to reach level X, for which you usually need at least a week. In addition to that, you must always install apps on your computer of dubious reputation.
Not recommended: You will lose a lot of time and it is unlikely that you will get paid.
If the applications are not recommended, the hacks much less. In the own page of FF Garena, they inform you that the punishment of using third party programs are: Lock your account and lock your device in a permanent way. So if you like Free Fire, take this into account.
Not recommended: You take the risk of they block your account and also your device.