Customising existing themes

I’m new to WordPress but have designed quite a few sites in Dreamweaver.

What I would like to do is take some of these fantastic WordPress themes and then customise them to suit some of my own designs and lay-outs. Can anyone recommend a simple article or guide available on the net to getting started doing this? I know there’s lots of material out there but they seem to assume a lot of knowledge.

I mean real step by step basic stuff like “you can change the banner by doing this and change the colours by doing this etc?”

Any suggestions (polite) would be appreciated.

Many thanks,