one way is to get a block of wood, the same thickness as the height you want the bottom of the label to be on the jar.
Line the jars up behind the block of wood, (or do one at a time with a small block) stand the edge of the label on the top of the wood and press one side back onto the jar. smooth round and it should be reasonably straight.
That was a neat gadget from Thomas Bickerdike (didn't it win a prize at the NHS?) but I wish the jar was vertical. I do as wessexmario says as it means the lid of the jar stays honey-free. It's easier on square or hex jars than round ones.
Remember, if the label carries the word 'artisan' you're allowed to have the label at any darn angle you want