The landlord can’t keep your deposit (or part of it) without your agreement. If you think it is unfair, challenge it.
So the TD scheme will write to you to say that the landlord wants to keep £X of your deposit. You can agree or disagree. If you disagree, it is on the landlord to prove their case.
So if you have received a notice that they intend to keep £X, just say no, and the process will continue from there. They should provide you with a detailed list of what the £X is for, and you can agree with some / all / none of it.
But again, it is for the landlord to prove, and if they don’t have really good records, including before / after photos from when you moved in / out, it’s really hard for them to prove any damage.
There is also a provision against ‘betterment’, which means that they can’t claim money to make the property in better condition than when you moved in. For example, if it had been painted 2 years before you arrived, and your kids have scribbled on the walls, they can claim for the cost of repainting less 2 years’ worth of damage, not the full cost of repainting.
The TD scheme protects tenants from unscrupulous landlords, so make sure you take your time reading and filling in the forms properly. There are time limits (possibly 21 days) but don’t rush.
Good luck.