How do I download photos my iCloud to my iPad?

iCloud Photos is a syncing service. Photos on iCloud are the same as on each device where iCloud Photos is turned on. If you don't have iCloud Photos turned on in the iPad but do have photos in iCloud you can turn it on as described in Set up and use iCloud Photos - Apple Support

If you had iCloud Photos turned on in the iPad and also had the Optimize Storage option on and want to change that so that your full size originals are on the iPad, turn on the Download and Keep Originals option that is described in the link above.

Another way that photos may be on iCloud (if you don’t have iCloud Photos turned on) is via a backup of an iOS device. The only thing you can do with a backup is use it to restore a device or set up a new device. A restore will erase all data (not just photos) currently on your iPad and replace it with data from the time of the backup. Restore your iPhone, iPad, or iPod touch from a backup - Apple Support