First off, soft subs are subtitles that are not hardcoded to the video meaning you can turn them off or on.

Now on to getting them to work...

Depending on where you get your source, the subtitles might already be in the directory where you movie file is located. If it's a Scene release then it's most likely in another .rar file where you'll have to extract the subs and copy them into your main movie directory.

~Before you do anything~
Download and install DirectVobSub.



ALTERNATE METHOD
Using VLC Media Player | Download Windows | Download Mac |
Step 1:
Open the file with VLC
Step 2:
Drag the subtitle file into VLC and it should work. Once again make sure the subtitle file and the video file are named the same.


For Mac users you can also try Cellulo
Read more about it here