Notice! The following tutorial has been made for WinFF-1.5.0 only!
In order to upload a webm file you will need to convert and eventually cut your video file to the desirable length. You can do this by using specific video editing software. The purpose of this tutorial is to enter you to the basics of this process.
First visit winff.org downloads page and download WinFF-1.5.0 (32bit or 64bit depending on your system). WinFF is a free software that allows you to convert, cut or edit your video files to webm format.
Install WinFF-1.5.0 and start it by clicking on the desktop icon.
Click on "Options" button on top right. This will enable the advanced settings like, time frame (desirable length of your video), audio settings, crop options etc.
Select desirable video format by clicking on "Preset" dropdown in the middle of the screen. In this case the right one is "Google WebM Generic Fullscreen" or "Google WebM Generic Widescreen" (depends on your video proportions).
Set desirable length of the new webm file by clicking "Time" tab above the "Preset" dropdown. Set start time by entering hours, minutes, seconds in "Seek to" dropdowns row. Set "Time to record" (this is the final length of your new webm file) in the dropdowns under "Seek to" row by using same time format.
Set destination folder for the new file by clicking browse button at the end of "Output Folder" field. Default is "My Documents".
Convert your new webm file by clicking on "Convert" button on top.
Review your new webm file by visiting the "Output Folder" and proceed to upload if everything is right.