Hi
We were in Paris a couple of weeks ago and I was dreading getting out to DLP on the RER, but it was sooooo easy, especially when I had had a day or two jumping on the Metro - I felt like a local!
Firstly you need to know that there are two types or rail travel - the RER (which is like our main line trains) and the Metro which is basically the same as the Underground and covers the city. Sorry if this is really basic, but once I realised this, it suddenly all made sense, I also twigged that the RER lines and Metro lines are not in the same place, but there are links and walkways from one to the other (doh!)
There are different RER lines, all letters, A, B etc., and Metros are numbers.
You can get a ticket that covers you for the day and will work on both RER, Metro and buses (bargain!). I wasn't brave enough to use the automated machines, so queued at the ticket offices
Thankfully everyone spoke great English, although I did try my school French!! Just explain where you want to go.
From DLP, the train goes only one way - it is a dead end station, so you just need to keep an eye out for your stop - luckily there is a nice little drawing on the side of the carriage that shows the stops and a little light lights up to show which is the next one!!! It is also announced, although I couldn't make head or tail of that, so I was peering out the window just to be sure (not good is it!?). It did take longer than I thought though, which made me a bit paranoid!
I think CDG E is a good starter, as when you come up out of the Metro/RER station, you are right there, turn around and you will see the Arc. If the queue isn't long it is worth going up here, rather than the Tower.
Don't worry about finding the Metro to then go on to Trocadero, they have blooming big signs above them, bear in mind, there may be more than one entrance to the Metro, but they end up in the same place under the ground.
It is worth getting a Metro map and you can see exactly where you need to go, always look for the number of the line i.e. 6 for the Trocadero and then where the train finally ends up, as this makes sure you are going in the right direction. However, Line 6 actually starts at GDG E, so you can't go the wrong way and Trocadero is about five mins. along. When I came up from the Trocadero, I couldn't see the Tower and had a bit of a panic, but just walk along the road a little way to the left and you will get a fantastic view.
Going back, get on the same Line, but make sure that it is heading in the direction of CDG and it will take you back to CDG E, get off and look for the RER A signs. They are pretty obvious too. You may have to walk along some underpasses and go up or down escalators, but don't worry - it is a web under there!
The RER A is the line that goes to Marne La Valee (DLP), but as others said, when you get to the platform, just check the overhead signs (several down the platform), that the light next to it is lit, as some trains branch off before there - I am afraid I also asked when I got on the train (you're getting the picture here now?!). You can't go wrong once on the train, as it won't go any further than DLP and you know you are there, because of all the pretty little girls in their Princess dresses.
I was worrying so much before I went, but actually it was really easy in the end.
Just be aware that the Metro does get extremely busy and everyone will push up together - just like London, if you are coming back early evening 5 - 6ish, be ready for the daily commuters.
and would like to go for a look again. I'm sure they can we easily persuaded to come with me to see the Eiffel tower 
at me haha. I don't blame you! We don't have trains or even a busy bus service up here so even when I go to Aberdeen or Glasgow I don't know how to 'work' the buses... I'll just have to learn the instructions 
I now feel like I can do it. I will just have to remember where I came from to trace my steps back to go back to DLP again 
Just like any other underground I suppose? Watch your bags and pockets etc 
and I know it sounds rude but don't engage in conversation with anyone trying to sell you stuff etc etc.
