Main difference is that in manual testing, a sapient, thinking processor (human) is looking at the results and can see "something is weird here".
Automated testing does not exist, it is wrong terminology, misnomer, even if widely accepted. In automated checking, totally dumb processor (your test script) checks for condition which programmer thought about, and blatantly ignores anything else, even if obviously wrong for a human manual tester.
What makes sense is automated regression testing (also a misnomer, it is checking again) when your automated scripts checks for known/suspected conditions (and ignores everything else, as all scripts do).
Human can do exploratory testing. Automated scripts, not so much :-)
But: to perform action in automated script, you need to know how to do it manually (and usually do it multiple times). You cannot automate something you don't know how to do manually.
Your automated test can check for small subset of what human tester would notice (so your manual tests are a good start). Some tasks (like pattern recognition) is beyond abilities of most automated tests, so you need to pick which conditions to check in script, and what will be ignored.
There are many cases where my automated tests perform action but do not check for errors, because I never seen any error in that action. So such test can silently fail to detect error, because programmer failed to anticipate it. To deal with such situation, you (programmer) have to use "defensive programming" techniques and i.e. check if page title is what you think it should (if not: something broke, but none of your checks detected it). This is not how human would detect such error, but that is all you have in the script.
And scripts are good in something humans are bad: Running exactly same checks day after day, very efficiently.