Select "File | Open..." menu item.
You are given a standard file open dialog.
Select the file you want to convert.
River Past Video Slice supports a range of video files. The complete list supported formats can be found here.
Once River Past Video Slice accepts the file, and display the first frame in its window.
Use the 
"Play", 
"Pause", 
"Stop", 
"Step Backward", 
"Step Forward" buttons, and 
slider to preview the video.
You can click on the time line to jump to the middle of the video. Note the cursor 
and preview window both change accordingly.
You can also drag the cursor along the time line. The preview window will be updated live.
