Compiled this in an answer to one question here.

There's no "how do I do X", and documentation is scattered, but we a lot of good stuff if one can find it. This is what I used the most:

One must understand the TX format & consensus rules:

• https://documentation.cash/ (still needs to be updated with new opcodes & optional TX format that came with the May '23 upgrade)
• https://github.com/bitjson/cashtokens

We have BitauthIDE as a kind of WYSIWYG native Script contract builder:

• https://alpha.ide.bitauth.com/ (check out the simple examples offered by the page!)

Then there's the higher level CashScript which should be familiar in syntax to those coming from Solidity, but program logic is actually quite different on Bitcoin-tech UTXO model:

• https://next-cashscript-playground.netlify.app/ (best CashScript editor for now, has auto-complete etc.)
• https://cashscript.org/ (check out the guides!)

And finally there are libraries which one can use to build apps that interface with the contracts:

• https://mainnet.cash/ (check out the guides!)
• https://libauth.org/

And indexers that can support the app:

• https://github.com/cculianu/Fulcrum (Electrum server, most wallets rely on it to find UTXOs belonging to addresses)
• https://try.chaingraph.cash/ (public graphML server, you can query the blockchain for whatever you want)

For swaps, I realized contract approach is probably better than the CoinJoin/PSBT technique, and I made a contract template which could be a building block for some DEX or something:

• https://gitlab.com/0353F40E/simple-swap/-/tree/master/

How to actually make an app that uses the contract? One example is the minting app that interacts with the Emerald DAO:

• https://github.com/mainnet-pat/emerald_dao

Finally, for token metadata, we have the BCMR:

• https://github.com/bitjson/chip-bcmr

and the first "tokenlist" registry is taking shape:

• https://otr.cash/