FTPS using Explicit TLS howto (Server)
First you'll want to create a certificate, this can be used in the Certificate Generator in FileZilla Sever. The Generator will want country code, state, city, etc.. This information doesn't need to be correct at all it is just used to generate the hash used to encrypt and decrypt the data being sent by the server and client.
Encryption strength for the certificate is chosen at the top of the generator: 1024bit, 2048bit, 4096bit. The bigger the hash encryption the more secure the data and account information will be. Their is however on thing that needs to be taken into account, CPU utilization. When you apply encryption to your FileZilla server the CPU will have to do many calculations to encrypt the data being sent and decrypt the data being received. Bandwidth will also play a factor in how much the CPU is being utilized. If you have a slower connection, lets say around 1.5mbps up you may not have to worry about CPU utilization as much. The best way to decide is to test. After you have created the Certificate enter its location into the "Private key file" field, or browse to it.
If your server has a direct connection to the internet the configuration is simple, check "Enable SSL/TLS Support".
Configure with NAT