It can be very dispiriting to pick up a fretless full of enthusiasm only to be shocked at the terrible sound you make from it. I know... I've been there! This bass guitar lesson teaches three exercises you can use to improve your intonation and start playing that fretless bang in tune.
The first two exercises involve playing major and minor tenths against open strings. In the third, you can play anything you like when you play to a drone. Here is the free download for drones in all 12 keys.
I have another lesson on playing the fretless in tune and you should give that one a watch too.
To learn how to get that beautiful fretless tone with effects head here.
Stick at it and play consistently and you will be able to do this in no time. For more practice tips, check out these lessons on how to practice properly.