To be honest the best method for migration is to migrate from the old computer's Time Machine External Hard Disk to the new computer. This has proven to be the easiest, quickest and most reliable method. Please click the Setup Assistant and Migration Assistant link for detailed instructions.
Remember in order to migrate the new Mac (in your case a MB Air) must and I mean MUST have the same or more storage as the old Mac has stored on it. So for example if the 2092 iMac has about 350 GB of data stored on it the new MB Air should be a model with 512 GB or more of storage. If your MB Air has less than what you have stored on the iMac then a migration will not work, why simply think of it as you cannot put 1 gallon a milk into a 1 quart container.
Please let us know if you have any questions.