I'm not claiming to be the master of winged eyeliner, but this method has helped me a lot to achieve even wings. I hope you find this tutorial helpful!:
I find the best way to do winged eyeliner, and to ensure that both sides are the same, using tape is the best method. I've tried it free hand, with a credit card, and with a spoon but the tape method is by far the easiest way.
Place a piece of tape under your bottom lashes in a diagonal direction up to the end of your eyebrow. I use bog standard tape but masking tape or washi tape (or any tape for that matter) is fine. Just make sure you make it less 'sticky' by placing it on a top or some jeans to pick up some lint. This way it won't hurt as much or pull on you skin as much when you're taking it off.
Now it's time to put on the liner. I use Soap and Glory Supercat liner which is a pen applicator eyeliner. I find that the pen type eyeliners are much easier to use (but you can use your desired eyeliner).
Start with the 'wing' of the eyeliner. Draw a line following the edge of the tape. Don't worry too much about going onto the tape as this will be removed later.
For a more dramatic winged eyeliner you can finish the tip closer to the brow, or by drawing a small triangle and colouring that in as the wing.
Next, draw a line across the lash line joining up to the wing. I keep the line the same thickness, but again, for a more dramatic look start thin in the inner corner of the eye and gradually make the line thicker towards the wing.
CAREFULLY remove the tape taking care not to pull too much on your skin. If anything needs tidying up dip a cotton bud into some make up remover and sweep any errors away.
And there you have it! Let me know if this tutorial helps you/if you try it. It'd love to see some pictures if you do.