I'm trying to sync an older iPad (IOS 10.3.4) with a brand new iPad pro 11. What's the easiest way to sync?

These support pages should get you started with setup of your new iPad - and the methods of transferring your data from old to new iPad:

Set up your iPhone, iPad or iPod touch – Apple Support

Transfer data from your previous iOS device to your new iPhone, iPad, or iPod touch - Apple Support

Use Quick Start to transfer data to a new iPhone, iPad or iPod touch – Apple Support

If the donor iPad, from which you are transferring your data precedes iOS12.4, QuickStart will not be available.

“Transfer” is rather a misnomer. In reality, your data and setup is merely cloned to the new device. Nothing is moved or removed from the original device.

An alternative is to restore either an iCloud or iTunes backup to the new device:

How to back up your iPhone, iPad and iPod touch – Apple Support

Restore your iPhone, iPad, or iPod touch from a backup - Apple Support

After setting-up your new iPad, having transferred data (if required) during the setup process - or restoring data from an existing iCloud or iTunes backup - you should ensure that you have enabled Apps and services that you wish to synchronise in iCloud, on both devices:

Settings > [Your Name / AppleiD] > iCloud - enable Apps and services as required

Do you mean transfer data from the old iPad to the new iPad?

If so then.

Transfer data from your previous iOS or iPadOS device to your new iPhone, iPad, or iPod touch - Apple Support