Carousel adding to Shopify product template

We require a carousel adding to the shopify template. A good example of this is the following link

https://www.harveynichols.com/brand/givenchy/265156-stars-printed-cotton-t-shirt/p3133292/

We know there are apps on the Shopify Market but we want it custom coded into our template. This can be done with flex-slider and the library is already installed in our site.

The Shopify theme is Debut but has been customised quite a bit.

Any more information, please contact me.