The stable version of nlstools can be installed from CRAN using:

install.packages("nlstools")

The development version of nlstools can be installed from GitHub (remotes needed):

if (!requireNamespace("remotes", quietly = TRUE))
   install.packages("remotes")
   
# remotes::install_github("lbbe-software/nlstools")
install.packages("nlstools", repos = c('https://lbbe-software.r-universe.dev', 'https://cloud.r-project.org'))

Finally load the package in your current R session with the following R command:

library(nlstools)