If your internet is “bad” you may want to hold off doing anything, if it will improve. “Bad” is not very descriptive. I installed KDE from scratch over a “bad” internet connection last week and ended up with a “bad” installation. Had to do it all over again after getting a “good” internet provider.
If you mean metered, then it is possible that Linux is not for you. Linux was born on the 'net, and everything good Linux-wise comes from the 'net. And a rolling release such as Manjaro needs connectivity or it just isn’t very fun. If your circumstances penalizes you too heavily, perhaps an up-to-date fixed-release might be preferable?
If you need to ask how to do it, then I suggest @eugen-b’s instructions will work, but can you work them?
I can only echo what most of the others have stated. Download the KDE ISO. You’ll have it on-hand from then on. And you might save the rest of us a ton of work trying to help you solve a fail.