Introductory material on splines I am looking for a basic, step-by-step introduction into modelling with splines.
(I have encountered splines while teaching another topic. The textbook I am using does not cover splines in sufficient detail for the students to be able to replicate things by themselves. I hope there is some good introductory material out there so that I do not have to reinvent the wheel and write my own notes on the topic.)

I think Semiparametric Regression with R, in particular Chapter 2 "Penalized Splines", by Harezlak, Ruppert, and Wand (2018) would work as a first introduction. The book is focused on practical implementation in $\mathsf{R}$ and accompanied by the $\mathsf{R}$ package HRW.
Probably Chapter 3 "Scatterplot Smoothing" in Semiparametric Regression by Ruppert, Wand, and Carroll (2003) would be a good addition.
