What is the best tutorial to convert a substrate-based standalone chain to a parachain?
There are a lot of tutorials on https://docs.substrate.io/tutorials/
Here is one to convert
In addition to the tutorials mentioned by Sam, this workshop is very thorough as it explains all the steps of taking your solochain to a parachain as well as building the front-end for it. It even ends with demoing the entire application.
Skip to this part to get an overview of the steps you'll need to take to convert your solo-chain to a parachain connected to a live relay chain.
You can check out this presentation which has Centrifuge's solochain to parachain migration as a case study (Sub0 Online: Storage Migration - Standalone chain to Parachain ): https://www.youtube.com/watch?v=sIgvyRFs-N4
Checkout the docs on how to convert a silo chain to a parachain: https://docs.substrate.io/reference/how-to-guides/parachains/convert-a-solo-chain/
Note that you must perform state migrations on your new parachain_starting from genesis_ to populate the desired state on the parachain. You cannot simply "flip a switch" on a running chain to become a parachain. https://docs.substrate.io/reference/how-to-guides/parachains/runtime-upgrade/
It is likely you need to evaluate your benchmarking closely and fit into the much more strick and limited weight requirements for parachain operation, compared to solo chain operation as well. You may need to redactor or when prune functionality based on those investigations. https://docs.substrate.io/reference/how-to-guides/weights/add-benchmarks/