Sweeping from Mycelium into Electrum:
1. Go to https://iancoleman.io/bip39/
2. close down the internet. You can also download the html, but still close down your internet after you download it. Run it on a computer with no internet or make sure it's completely shut off.
3. enter in BIP39 Mnemonic
4. let it calculate
5. Take the first private key under derived addresses
6. Sweep it into Electrum
7. Broadcast it.
At this point the "offline computer" represents a security risk the minute it is turned back onto the internet.
At the very least check the actual code before you run it; better methods exist, depending on how secure you want to be.