You simply get an "on-the-go" (OTG) cable (like this:
https://www.amazon.com/Micro-USB-OTG-Adapter-Cable/dp/B00D8YZ2SA) from Amazon, eBay, AliExpress or wherever and connect your hardware wallet to your device.
Mycelium works with Ledger and Trezor and will "import" a watching only version of the hardware wallet (just like Electrum desktop does). You then create the transaction and when you want to send it, it will get sent to the hardware wallet for confirmation and signing... Just like when you use it with a desktop wallet.
You may want to check if your Android device supports OTG first, some manufacturers and some devices do not support it.