To post the video onto a homescreen thread, the video must:
- be a WebM file
- be under 3072 KB
- be no longer than 120 seconds
- have maximum resolution of 2048x2048 pixels
- have no audio
More likely than not your screen recording app of choice will not be in the WebM file format. You will need to convert the video accordingly.
ffmpeg, a command-line utility, is one possible tool; see this guide here. Otherwise online conversion tools are available, such as Online-Convert.
To frame your screenshot in a device mockup, Screener offers the easiest solution. More customizability would entail using Photoshop and finding suitable .psd files.