On a page model:

Implement as a ForeignKey relation, same as wagtailimages.

from django.db import models

from wagtail.wagtailadmin.edit_handlers import FieldPanel
from wagtail.wagtailcore.fields import RichTextField
from wagtail.wagtailcore.models import Page

from wagtailvideos.edit_handlers import VideoChooserPanel

class HomePage(Page):
    body = RichtextField()
    header_video = models.ForeignKey('wagtailvideos.Video',
                                     related_name='+',
                                     null=True,
                                     on_delete=models.SET_NULL)

    content_panels = Page.content_panels + [
        FieldPanel('body'),
        VideoChooserPanel('header_video'),
    ]

In template:

The video template tag takes one required postitional argument, a video field. All extra attributes are added to the surrounding <video> tag. The original video and all extra transcodes are added as <source> tags.

{% load wagtailvideos_tags %}
{% video self.header_video autoplay controls width=256 %}

How to transcode using ffmpeg:

Using the video collection manager from the left hand menu. In the video editing section you can see the available transcodes and a form that can be used to create new transcodes. It is assumed that your compiled version of ffmpeg has the matching codec libraries required for the transcode.