Homebrew’s website:
https://brew.sh

Install with an account with admin privileges if possible. If that is not possible at all then you can go down this rabbit hole (if not be prepared for permissions issues):
https://superuser.com/questions/619498/can-i-install-homebrew-without-sudo-privileges

based on this (untar anywhere):
https://docs.brew.sh/Installation#untar-anywhere

Normal installation procedure:

Install command line tools if you don’t have them:
xcode-select --install

Launch the terminal, then copy this command and press enter, homebrew will install (this command is found on homebrew’s homepage)
/bin/bash -c "$(curl -fsSL https://raw.githubusercontent.com/Homebrew/install/HEAD/install.sh)"

Now install svn by typing this command into the terminal:
brew install subversion

Common installation issues can be found here:
https://docs.brew.sh/Common-Issues

On macOS it usually involves reinstalling the command line tools after a major OS upgrade, so:
xcode-select --install

Then:
brew upgrade

Uninstalling homebrew will fix a FUBAR install:
/bin/bash -c "$(curl -fsSL https://raw.githubusercontent.com/Homebrew/install/HEAD/uninstall.sh)"