I ran out of time last night to post the actual progress so far.

First stage the plates:
Using tectonic plates to draw a world map by dhalsimrocks is a very useful tutorial for this stage. I drew eight random plates.

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I picked one a jagged section of plates to run the whole length of the world moving apart. at the opposite boundary i had the plates coming together.

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The Green lines are where plates are spreading apart at constructive (divergent) plate margins and the blue lines at destructive (convergent) margins. The unmarked lines will either be transform faults or inactive.

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The next stage was a lot of trial and error creating a new layer and just sketching with a large soft green brush to get the land areas in the right places. to give a balance of land and sea.

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I wanted the land to cross the plate margins because that gives the more interesting features. I had in mind (very generally):
constructive margins at sea create mid ocean ridges
constructive margins on land create rift valleys

destructive margins at sea create island chains
destructive margins on land create large mountain chains
where ocean plates meet land plates it forms mountain chains

Based on this and my land and plates i could add the major relief features.
Brown are the mountain ranges and the light green rift valleys.

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I'm going to tweak the land masses slightly no doubt but these will do for the moment.