If you want to do it as you propose you will need some software like ghost that can clone one partition to another. Catch is in most cases the partition has to be the same size, so if you are using a 1TB drive for your OS there will be additional steps required to clone it to a 256GB SSD.
There are programs that can re-size a partition, but they run the risk of corrupting the partition and making it difficult to get it back.
I'd also recommend a new OS install to the SSD (with the HDD unplugged), reinstall all applications to the new OS, then copy any files you need from the old HDD to the SSD (and external backup), before formatting the HDD and using it as a storage drive.