There is no specific method to frame the trap in Free Fire. Instead, there is the way to do so in other games, especially fighting games like Street Fighter. The method to farm trap is similar in every fighting game, so feel free to try this method on Free Fire.
The term frame trap refers to the moment when you attack to hit or block stun your opponent, then hit them with another move right after hit stun ends, so that if they try to attack in between your two moves, their move will get a counter hit. The basic way to do a frame trap is just to hit your enemy with something plus on block, and then hit them with a reasonably fast follow up move. Doing a plus on block move, then a follow up more will frame traps in basically every fighting game.
There are three basic ways of frame trapping. The first one is hitting with a move that is plus, then following up with another one. The second one is hitting with a move that allows you to cancel it later, and then cancelling late enough your enemy has a brief chance to press a button. The third one is canceling from a low hitstun move into the one with more startup, like chaining light to heavy in chain games.
For those who want to get technical, you are able to calculate how small the gap between your first and second move is to see how tight your frame trap is. If your first one is +3 and your second one has 5 startup, that is the gap of 2 frames. They would have to use a move that comes out in 2 frames to trade with you. If you are doing a cancel like the third method, you can just compare the blockstun of the first move to the startup of the move you are cancelling into and that is the gap. In the case of delay cancels the math is able to be a bit fuzzier, because you are allowed to cancel whenever you want, so the gap can be bigger or smaller depending on when you cancel.
A true blockstring is the thing you want to avoid. It is where your second move hits the enemy before the blockstun ends. If they do not exit blockstun, they do not have a chance to press a button. If they do not have an opportunity to press a button, then even if they are smashing, they will continue blocking, and you will not get the chance to open them up.
Aside from that, frame traps can also work based on hitting your enemy with something faster than any of their likely options.
In Free Fire, the frame trap has something to do with the weapon. Your priority is to pick up a weapon as soon as you touch down. A few players usually dawdle behind walking through the greeneries, looking for the big compound or city to loot and wasting time in the process. It is a bad idea as you will often meet enemies that landed before you and already have weapons.