hey bro, this is easy!^^
ok, you are beginer but anyway, it is logic, when you insert you must remplace, for example the grass you insert where was the original grass tile, the tree there where was the first tree and so on.....and for the pallete you need first to fit the colours of the actual pallette with paint or other so that you may insert without any colour problem, then when it is inserted you change the pallette like the tiles colour you inserted....
and for houses it is more difficult but it is still logic that you will not insert a house in the same tileset as the one of the grass coz then the pallette will not fit at all......
but well, you know, a few month ago I was already creating tiles but I couldn't insert them, when I first started learning it seemed so difficult but now tile inserting is no problem at all, and with pallette adjustments I also got no problems, I can adjust all the pallettes of every tileset so that I may insert above 1000 different tiles with all different colours and so^^
but well, it will come alone with doing it every day ;)
no I should go, bye