I am travelling right now, and only occasionally have a godd internet connection, so I probably will not be able to respond in a while.
I have cleared room on my hard drive. I have a Time Capsule, so everything has been backed up into Time Machine before I start anything. Just to be safe!
That is good.
In Aperture I will switch to the Aperture Library (which at the present moment is blank, I am using my iPhoto library in it). Then I will go to File > import > Library and choose the iPhoto library under the Pictures tab. And let it import. After this is done (how long will it take at 240 or so GB?)
If you have not repaired your iPhoto Library for some time, it would be safer to repair the library before importing, see: Repairing and Rebuilding Your Aperture Library: Aperture 3 User Manual
How much disk space did you clear? Twice the size of your iPhoto Library would be good.
After this is done (how long will it take at 240 or so GB?)
That is hard to estimate. be prepared for more than an hour wait.
I will then do another backup. ? And play around a bit to make sure the Aperture library is fine and useable in both Aperture and iPhoto.
And then I am ready to delete the iPhoto library. But *exactly* how do I do this? Go to Finder > Pictures and right click/delete on the iPhoto Library folder? Or do I need to do this within iPhoto itself?
To delete the iPhoto Library drag it to the Trash and empty the Trash.
As the second backup I'd recommend to keep an exact copy of your iPhoto library on your backup drive. This way, you do not need to restore the library from your TM backup, if there should be any problem.