Click on the button (with the points) right of the window.
Select the SSA files
Choose the desired codec (normally the same as the movie).
It is better to encode in 2 passes. Indeed, in 1 pass, the encoding will be done in CBR (Constant Bit Rate), the fast and slow sequences are encoded with the same amount of information per second. The 2-pass will be made in VBR (Variable Bit Rate), ie the rapid sequence will be encoded with a high bit rate whereas the slow sequences with bit rates will be lower.
Click the button "Configure". In the new window, choose "2 pass - first pass".
Click OK and then OK
File>> Save as AVI, give a different name and let the process unfold.
When the first pass and finished, you get a file containing some informations about the bitrate.
Video> Compression>> Sets
Choose 2 pass - second pass and start a new backup under another name.