Be aware that new 3ds, particularly the xl are a lot more temperamental with the downgrade, and if it fails you will be forced to update to 10.6, which makes normal downgrading impossible. People report that updating it to 10.3 before downgrading it to 9.2 increases its success rate.
If it does come to the worst, you can still get it hardmodded and downgraded, but you will have to spend another $20-30 to get it done. GBATemp has a list of trusted hardmodders.
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As for how to do it, search the more or less catch all tutorial thread. That will guide you through the whole process.