The RSPB web site

This gives instructions for a variety of water/damp environments for wildlife - a small container pond, a larger pond, a bog area and damp ditch:
https://www.rspb.org.uk/birds-and-wildlife/advice/gardening-for-wildlife/water-for-wildlife/making-a-pond/

 A straightforward video for a medium-sized pond that keeps it simple:

https://www.youtube.com/watch?v=56bacePG8hA

Videos on making a container pond:

https://www.youtube.com/watch?v=pVXnwvv_tio

https://www.gloucestershirewildlifetrust.co.uk/garden-ponds

https://www.youtube.com/watch?v=z7OJ8vtOb3c

Leaflets (could be printed off if required)

A couple of comprehensive leaflets on creating and maintaining a medium-sized pond:

https://freshwaterhabitats.org.uk/wp-content/uploads/2013/09/Creating-Garden-Ponds-for-Wildlife.pdf

https://www.wildaboutgardens.org.uk/sites/default/files/2021-03/Ponds%20booklet_Web%20spreads_0.pdf

Other pond sites

Wildlife Trusts on how to build a range of things, not just ponds:

https://www.wildlifetrusts.org/actions

Royal Horticultural Society:

https://www.rhs.org.uk/advice/profile?PID=622

Gardeners' World (magazine) video - good for explaining what role different plants have:

https://www.gardenersworld.com/how-to/diy/creating-a-wildlife-pond/

For a more 'family story' description for building a medium-sized pond:

http://www.thesmallgardener.co.uk/build-wildlife-pond-part-1/