Just the raw numbers? You list the y's and the x's and the model is approximating y=f(x) from the above example. You can totally do it with pen and paper. This is what it'd look like (for linear regression): https://observablehq.com/@yizhe-ang/interactive-visualizatio...

You can write an f(x) and record the input and output and that can be your training data. Or just download some time-series data or something.