How do I build and review a Bitcoin Core GUI pull request? I am a designer and I would like to start reviewing pull requests (PRs) in the Bitcoin Core GUI repository.
How do I build the adjusted Bitcoin Core GUI in the pull request? I am used to just double clicking on an application icon to open the GUI.
It is difficult to answer this question as I don't know what your current skillset is. You will need some basic command line and Git experience and you will need a GitHub account. I will assume these as prerequisites for the sake of brevity.
(There is a great free introduction from Tracy Osborn to command line here. There is a free Codeacademy course on Git here.)
I will take an example PR from the Bitcoin Core GUI repo (PR 86) which was opened by hebasto, a Bitcoin Core contributor, adding a Tor icon to the GUI. At the time of writing (September 2020) it is open but it will hopefully be merged in the near future. Once it is merged hebasto may delete this branch (200902-tor) but this example will be illustrative regardless. You need to locate and build the branch that the PR author has been working on.

To build hebasto's PR branch (200902-tor) you can click on it and then you will be on hebasto's fork of the Bitcoin Core GUI repo. You will need to clone hebasto's fork. You do this by clicking the green Code button and then clicking on the link you need to clone. In this case it is https://github.com/hebasto/gui.git.

Once you have the link you need to clone go to your Terminal and use the git clone command. In this example it will be:
 git clone https://github.com/hebasto/gui.git 

Before you clone it you can choose where to clone it. Using mkdir and cd commands you can make a new directory and change to that directory before you run the git clone command.
Once you have successfully cloned hebasto's fork you can checkout hebasto's PR branch (200902-tor) using:
git checkout 200902-tor

Now you are ready to follow the Bitcoin Core build instructions for your operating system e.g. (MacOS, Unix, Windows etc)
If you are building the GUI remember to use the --with-gui flag when you ./configure.
For more detailed guidance on reviewing Bitcoin Core PRs see this excellent post from Jon Atack.