Hey so I have a 2000 toyota corolla 4A-FE manual, its had connection problems in the past so got a new battery and it didn't start one morning so I tightened the terminals (and put fuel in it) and its been fine after that. But since there looks like corrosion on both I want to replace them and its cheap and easy. The problem I have is I'm confused on how to cut the positive cable and whether there will be enough wire left to reach afterwards:
That the car now cranks and runs correctly tells us that the battery and the cables are conducting electricity acceptably. I see no need to cut and refasten either cable, especially (as you note) the (+) cable may not be long enough to allow connection after it's made shorter.
The minor corrosion visible in the photo is external to the clamp surfaces, shouldn't be a concern, and you don't need to wrestle with the too-short-cable issue. If you want to make things cleaner, disconnect the (-) cable, then clean the (-) post and the inside of the (-) clamp. Leaving the (-) cable disconnected, then disconnect and clean the (+) connection parts. Then reconnect the (+) cable, and finally reconnect the (-) cable.
From the images I'm seeing very little corrosion. I think your bigger issue is the clamps themselves seem to be worn out. While you say it is "cheap and easy", you also ask "where do I cut them at" ... the answer to that is, you don't. There is the nut on top which is there. This, when removed, separates the battery cable from the clamp. You can replace the part between the cable and the battery post without having to cut anything. Just replace the clamp itself, then reattach the cable to the new clamp. Very easily done and should solve all of your issues.
Also, unless you're replacing the battery, you should use these same nuts to disconnect the battery from the car. This will save wear-n-tear on the clamps and is actually easier to accomplish.