Basically speaking, you insert the spool from the left, depressing the spring that applies pressure to keep the spool engaged with the drive chuck, on the right. see below

As you move the right side of the spool towards the drive chuck with your left hand, raise the lever (if one exists) protruding from the back of the spoolbox so that it will rest on top of the spool -in the groove in the spool. Then proceed to engage the drive pin on the spool with the drive chuck.

FYI, that lever (if it exists on your piano -it doesn't exist on all Aeolian players) is what is used to trigger the Automatic Shut-Off device. In units that are equipped with Auto Shut-Off, the lever is lifted out of the groove, in the spool, by the paper on the Take-Up Spool. This action keeps a pallet valve closed during the Play and Rewind cycles, OR it keeps a microswitch in the 'On' position during the Play and Rewind cycles. (Aeolian had two arrangements for Auto Shut-Off.) In both cases, after the roll is completely rewound and the lever falls back into the groove in the Take-Up spool, the power to the electric vacuum pump is turned 'Off'. Here's a picture of a unit with the microswitch type auto-off feature. see below

Also of interest is the fact that Aeolian had two different arrangements for systems with the pallet valve controlled auto shut-off. Both of them involve a rather heavy metal lever. Here's one of the two:

and here's the other:

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