In case you're using blockchain.info, here's a rough sketch of how it's done best:

- Receive money
- new address - call it "Mastercoin"
- Copy address
- Send money --> quick send - to this address  - send X + 0.005
- Send money --> Custom
- From: Mastercoin
- To: 1EXoDusjGwvnjZUyKkxZ4UHEf77z6A5S4P
- BTC: X
- Miner fee: 0.005
- Review payment
- Check on 1exodus