Step By Step Guide "Mummy would understand"...

If you mean without physically disconnecting the cables then...

Reset (or when you power on the Amiga) hold down both mouse buttons until a screen called "Early Startup" appears...
Then click on "Boot Options" (left mouse button)
Now on the screen that's appeared using the left mouse button click on all the items that say "Enabled" to change them to "Disabled" except for the one that says "DF0" (leave that one at "Enabled")
Then click on "Use" and then click on "Boot"
It might also be handy to already have the floppy disk you are going to use already inserted in the floppy disk drive...
