Before you upload your video, you must ensure they are clipped into sections you would like using. You cannot send full games, large sections of games or extended periods of play. Only the clips of video you would like to go into the video can be sent. This can be done on your phone/tablet very easily using the steps below. It is the same process on all iOS devices and will be similar on Androids.
On the video clip, select the edit button at the bottom or in the top right on older models.
Slide the yellow bars to adjust the clip to the section you want. If you press play in this window, it will play your selection.
When the clip is the correct length, press done. You will likely want to press "save as new video" otherwise you will lose any other video on the original clip.