I've now done this twice to my carbon legged gitzo(I'm assuming yours are carbon)

Lets say you start with the outermost(or upper) leg first:
You undo the twist lock all the way out till they drop down.(assuming that you're holding the tripod the right way up)
As you pull the leg out, at the very top of the leg(but inside) will be two halves of a white plastic bush pair(they'll probably fall out as you remove the inner leg form the outer leg(no problem).
The white plastic bushes wil be covered by a grey greasey substance, and so will the threads on the twist locks. That stuff is sticky stuff, and helps to keep the inner bushes lubricated, but as you touch the leg tubes with your now greasy hands, the grease gets al over the place

Each leg operates in the same way!

Replacement is the 'hard part' when you don't know what's happening,and in trying to replace the legs back into their respective tubes, I broke one of the bush halves.. ooops!
Then I realised what I did wrong, and you need to look carefully inside the inner leg tubes to see how they go back.
Each bush is an identical half, so they can go back in any orderm but you'll notice that when you place the bush halves around the leg again(they only go on in one way!!) they don't meet each other and they form a straight gap about 5mm wide. That's the important part to watch for inside the leg tube.
look inside and you just notice a faint line the same thickness as the gap between each side of the bushes(where they don't touch!).
That gap in the bush has to line up with the long notch inside the leg tubes.
This is how/why the leg doesn't rotate inside the inner tube.
The bush is statically fixed and located onto each outer part of the leg with a circular shaped recess(on the leg) and a similarly shaped raised circle on the inside of the bush. They only go back onto the leg in one way and fit tightly. So watch out for the gap in the bush that matches the long notch inside the leg. If the leg fitted with the two bush halves doesn't slide back in nice and smooth without any force, then you've fitted them the wrong way.

So what you would be doing to fit them back together again is;
* have the bushes back on the leg and sitting back at the start of the inner tube.
* making sure that the bushes are fitted nice and snug in their correct spots on the leg.
* have the gap between the bushes lined up roughly equal to the long notch inside the inner tube.
* wriggle this way and that way and with very light pressure inwards on the leg being fitted it will find it's location position and then slide in nicely, and the leg should sit there near the top of the inner tube without sliding in(if you're now holding the tripod upside down.. which you probably should be! )
*did you remember to fit the twist lock back onto the leg before you placed it back inside the inner tube

Note: there are no metal parts in the system at all
What I now do if I ever get much in the gaps in the legs, is to vigorously wash them in just about any cleanish water I find.. even if it's salt water. Salt water(AFAIK) does not affect plastic carbon fibre and any other material in the leg assembly. It may leave a white residue sometimes, but I've found that both easy to just wipe off with a wet rag, or to clean off if there has ever been a build up of the stuff.

My only dislike is the muck, mud and sand build up which accumulates around the twist lock threads, and into the gaps between outer leg and inner leg.
If your tripod has the stainless steel screw in hook in the centre pole, then that's what you want to keep out of salt water. Even though it's stainless, it still oxidises!!(watch that).
Mine has some rust spots starting to appear.

Even though I have broken one of the inner white bushes, that affected leg still operates perfectly. The breakage wasn't complete and I replaced a part of that bush back on the leg and it still works as intended. I don't know if they are parts that you can purchase as service items. Have forgotten about it till this thread.

Hope that helps, if not and you need pics, I can upload some tomorrow night if you like.

If it sounds hard, it's not. The only hard part was figuring out how the leg with the two bush halves went back into the inner leg tube, and I tend to learn from my mistakes
Just don't type back with any replies whilst you're pulling it apart and putting it back together again. the grease is hard to get off your keyboard!