Here's a step by step guide:

- make yourself a bitcoin testnet wallet (online wallet, mycelium testnet for android, etc.)
- grab some testnet bitcoin at a faucet
- install the Byteball TN wallet
- click the link for the transition bot on byteball.org, it's under the download links
- transition bot will start in byteball wallet and will guide you
- send the exact bitcoin amount to the bitcoin testnet address the bot tells you
- wait for confirmation (the more bitcoin fees you add to the tx, the faster it gets confirmed)
- the bot tells you your balance, probably not enough, because your bitcoin wallet moves the change amount to a new bitcoin address. In this case, you have to transfer the complete amount of bitcoins in your wallet to yourself. The bot tells you from which bitcoin address your payment came in, check if that is your address and if you're sure that this is your address, send your complete bitcoin balance there
- wait for confirmation of this tx, the bot will update your linked balance (you can enter any text like "Hi" for an update from the bot)