This is the easiest way.
- Use a centralized exchange (like Kraken, Binance…)
- Buy LTC, BCH, anything you want
- withdraw that
- Swap (exchange) those through a swap service, from the crypto you bought, to XMR.
- https://trocador.app/ is an exchange aggregator that’s privacy friendly and Monero friendly if you want. They don’t take any cut and you can get better margins this way.
- Optional but I recommend it: churn your crypto by sending all of it to yourself multiple times, waiting a few days between each churn. Easier to do with Feather Wallet on desktop as it has coin control. This minimizes the links to your identity: Monero shines through multiple transactions, but tx to tx can statistically be liked. The more you do, the blurrier it gets.
- On Feather Wallet (which is open source), there’s a "coins" tab where you can see each unit of crypto you have, and can "sweep output" to "yourself (churn)" easily.
- Spend your XMR, have fun :)
Additional notes:
The initial crypto should be a relatively famous one with low transaction fees so you don’t lose a lot in the process. The two I wrote are nice.
For swapping, the higher the amount, the lower the fee % will be. Always try trading over $20 to minimize fees.