Select a liquidity locking platform: There are several platforms available that provide liquidity locking services, such as Team.Finance, Unicrypt(UNCX), and TrustSwap.
Using any third party is a recipe for disaster. There have been dozens of platforms which offered such services, staking, yield, liquidity generators, and so on, which have either scammed or gone bankrupt and the users have lost everything. Just don't do it.
The timelock can only be cancelled if when you are broadcasting the timelocked transaction (that is RBF-enabled!), you broadcast another transaction without the timelock and place a higher transaction fee on it.
The timelocked transaction cannot be broadcast until after the timelock has expired. You can invalidate it with a transaction paying a lower fee prior to this, since the rest of the network know nothing about the timelocked transaction. The original transaction also does not need to be RBF enabled, since again, the network knows nothing about it.
is it OK to use the coinbin online?
Absolutely not. You should download it from
https://github.com/OutCast3k/coinbin/, verify the hashes against the provided file, and the run it on an airgapped device.
If you plan to create an address using OP_CHECKLOCKTIMEVERIFY (which is what coinb.in does), be aware that any coins sent to that address
cannot be recovered until the timelock has expired. There was a recent case discussed here of a user who accidentally locked $15,000 of bitcoin for 125 years, and there is nothing anyone can do to get it back before then. I would strongly suggest creating a test address with a timelock in the near future first, funding it with a very small amount of bitcoin, and double checking you can spend from it successfully, before you do anything with a large amount of coins.