I'm sure I've seen instructions on how do do this unfortunately it was a few months ago and have forgotten them.
Thanks in advance to anyone who answers.
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Assuming you are using Synaptic Package Manager, in your text file, you structure it in the following format:
where package name e.g. gimp would be a space or tabs can be install, remove, purge
And obviously put each package on a new line. No need to include dependencies.
Then in Synaptic, go to File > Read Markings and point it to your file. What this does is mark all the changes to be made and then you can proceed as usual. I have not tried @Jo-erlend's method but it definitely sounds simpler