How do I put my iPad in discovery mode?

What, specifically, are you attempting to pair with your iPad and iPhone via a Bluetooth connection?

Whilst it is technically possible to do so, be aware that Bluetooth pairing between an iPad and an iPhone is generally pointless. If your intention is to use Cellular Hotspot features of your iPhone, this link is optimally achieved using the native WiFi hotspot connection - this being a largely automated process. Hotspot connections over WiFi offer substantially greater throughput, range and reliability.

How to set up a Personal Hotspot on your iPhone or iPad – Apple Support

If you still wish to pair your iPad and iPhone over Bluetooth, you first need to manually enable the hotspot feature on your iPhone:

Settings > Mobile Data > Personal Hotspot > Allow Others to Join - set to ON

Next, Bluetooth for both the iPad and iPhone must be placed in Discovery mode:

Settings > Bluetooth (accessing this page will automatically place the device in Discovery mode)

Whilst both iPad and iPhone are in Discovery mode, you can mutually pair the devices.

Again, I must stress, unless you have an unusual “edge case”, pairing of iPad and iPhone over Bluetooth has no benefit.

What device are you trying to connect? Typically it is the device that needs to be in discovery mode, not the iPad. With the iPad, you just need to be on the Bluetooth Settings page at Settings > Bluetooth.