There is a full kit or components available from https://shop.pimoroni.com/products/my-naturewatch-standard-camera but if you already have a spare Pi or even a Pi plus a camera then you may need even less components.
You can use the instructions located https://mynaturewatch.net/
Or alternatively if you have a 3d printer/ use the fab fridays at pontio I have designed a 3d printed case that you can get from https://www.thingiverse.com/thing:3242776
This is a great thing to build with kids and you should be able to get some brilliant pics of creature visiting your garden. It would not be too tricky to extend the software as it uses OpenCV to classify the images and use Lora or 2g to send notifications when your camera has taken a picture.