Open the video file you want to work with by clicking the nice big button on the upper right or by dragging and dropping a video file onto the app. You can also setup Windows to Open video files with the app.
To control the video position, drag the video thumb slider. The 2 buttons on the side will jump 15 seconds in either direction.
Use the Start and End buttons to mark the position of your final clip.
Use the buttons in the Quick Clip section to clip the selected duration from the current video position.
Some sites have a limit on the FPS (Twitter has a limit of 60fps) so if you want to make sure the output video doesn't cross the limit, check the check box, and enter the limit of the fps in the text box.
As with framerate, video bitrate is also another thing that some places might limit. To limit the output video's bitrate, check the check box, and enter the limit in the text box. The button will reset the value to about 22Mbps.
Sometimes your video could be letter boxed due to how the video capture works. Press this button to attempt to detect where such borders are and they'll be cropped out of the video.
Select what type of hardware acceleration you have, any form of acceleration will be faster than none.
Press the button and the output video file will begin to process and it'll be in the same location as the original file but with the word trim and some unique values added at the back. The Test Mode check box will just create the FFMPEG parameters and put them in the clipboard... I guess you'll know if that's useful to you.