I think the key is trying to get a blank page in your head. Not to even think of existing tunes, styles or even types of tune, as a starting point.

Doodle along until you find a sound or a pattern that pleases you. If you find or fear that what you have come up with is similar to something else, just alter or doodle along around those bits and you will eventually come up with an original tune.

It may be a fairly rubbish tune, but if you file it away and carry on, I am sure that you will be able to build up a collection of tunes, which will then make it possible for you to choose the better ones and discard some earlier efforts. Not that you first tunes should automatically be rubbish.

Don't discard things that seem too obvious for they may only be obvious to you. That is your style, don't be afraid of developing this.