Textures:
Filters->Map->Make Seamless in Gimp
Some patterns in Filters->Render have seamless options
Noise for uniform textures or variation for other textures
Clone Tool and brush, also various colour options, layers modes etc.
I can't really give you step by step on how to make texture, every one is different. A lot of photos from CGtextures.com is nice enough to use without too much tweaking.
Rendering tile in blender:
http://www.gamefromscratch.com/post/2015/11/20/Creating-Isometric-Tiles-in-Blender.aspx
Stop on the step when you have flat tile in camera region. Use Cycles and your texture with Emmision shader set to 1. You can ignore lights etc. because this is giving you your texture in correct perspective. Render it twice as big as your tile and then downsize, it usually looks better this way.
You could just rotate your texture by 45 degrees and then resize to tile's size, but it's going to be a bit blurry.
Making tile:
Grab some tile template from internet (recolor it to some bright coloursif you want), open it in Gimp, paste your rendered graphic in the layer on top. You can copy and merge your tile a couple of times so transparent pixels on the edges become less transparent. Select areas outisde the tile on the template, then delete them from tile. Then fill empety pixels on the edge of tile following your template. I'm using Pencil tool and pick some random colours from tile. Also paint over the too bright and too dark pixels on the edges. Save as png.
Upload your tile to the Tiled.
http://www.mapeditor.org/ Use it on the map, if it has ugly parts go to your file in Gimp, use brush and clone tool to polish your graphics, then reload tile in Tiled.
Transition tiles:
You can make transition texture and repeat other steps. For simple textures open two tiles in the layers in Gimp, then use layer mask on top one and mask half of it. You can mask middle of it or make some scratches and patches.
Scenery:
https://www.youtube.com/user/AndrewPPrice
https://www.youtube.com/channel/UCG8AxMVa6eutIGxrdnDxWpQ
iirc they have some easy nature and rock tutorials. For more compicated stuff you need to learn Blender or follow some other tutorials.
Render in Cycles with the same camera as tile and simple lamp setup, even only sun lamp in the angle that produce short shadow. Then edit and overpaint in Gimp or other program. Try to render it in the size you need or twice as big, if you resize it by hand it will be blurred an full of artifacts.
Walls:
Make or download template and use it as background image in Blender, then either build it from 3d objects (planks, bricks) or just make vertical plane and render texture like in the case of tile. if you are making it from 3d objects copy your rendered picture layer on top of template and delete area outside wall. Also you need to experiment with lights, like using horizontal lamps perpendicular to wall's surface. Overpaint it a bit, tweak colours and levels. Check in Tiled and edit.
creation:
