Find the Tilesets

Before you can do this you'll obviously need to location two or more Tilesets you'd like to combine, as this is a tutorial and not for practical use I've selected the RTP 004-Mountain01.png and 015-ForestTown01.png as seen below:
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This just isn't any old choice, I find that the town set would be very flat without elevations, some sort of approach and surroundings. The tiles themselves are of the same style and won't clash.

Warning

Basically the best sets that work together are either made by the same person or people that have added onto a style. Hence why alot of people opt for the entire RTP look, though it doesn't look original it flows.

I put together an image to show how it would be obvious that the sets clash, basically the same three items onto a grass background. Here's the worst flow example:


The items are the same but it's the difference in colours, shades / highlights, saturation and from looking at it closer there's a completely different style of pixels going on in that tree. The RTP looks bubbly, bright and soft where as the other set looks serious, sharp and dark.

If you were wanting every one of your sets to flow to the best they can then a complete re-sprite would be necessary or edit the second one with the objects from the first keeping a close eye on how the second ones are sprited.

Notice

Guessing that would be a complete pain for most people, which is why it's easier to select a 'set' (As in complete multiple tilesets) from one genre and or series.