Very often, your projects are completely unique, so you start from scratch and begin chugging away.
Sometimes however, you have a TM from a previous similar project, so you can just plug it in and start translating.
And sometimes, a client provides you with previously translated documents that you may be able to use if you can covert them into a TM.
This is essentially a time consuming effort, so make sure that it is worth you time.

OmegaT does not have a native aligner, so we will use another CAT tool called Felix.