If you are building a ring wall or are installing a rainwater tank in a challenging location, making a template of the rainwater tank base makes life significantly easier.
Fortunately, I had a piece of fibre board from a deceased book shelf. You will need something light but not overly flexible - cardboard will work.
To get the diameter of the rainwater tank, use a flexible measuring tape to measure the circumference (around it). Then divide by pi (3.14159). Ideally you will want a panel at least this size, but if one side is too narrow - like in my case - I can show you a workaround later.
Measure the halfway point on all four sides then use a long stick to mark the centre.
Screw a screw into this centre point. Tie a string to the screw then a pencil to the end of the string. The distance from the screw to the pencil tip will need to be half the diameter you calculated - which is called the radius.
Draw a big circle with the pencil tied to the screw.
Using a jigsaw (preferred) or a keyhole saw, cut along the circle you drew.
If necessary, nail or screw a stick so you at least know the extent in both directions. Cut the stick to the same length as the diameter and I would recommend at least three nails or screws.
Finished! Next is using it.