Well, there are several methods.
Method one, no planning. This is how most 3rd party maps are made. you just open radiant and start mapping hoping to take the ideas inside your head and make the next casa. Tohunga, Facade were made like this. 1/50 times you do this, you could end up with a map which gets played lots.
Method 2 is the method which I use now and which I recommend.
* Think of a theme, any theme what so ever. Lets just say our theme was a carnival.
* Now get out a notepad and planning begins. You need to first think of the atmosphere you want, should it be happy and bright blue skies, or should it be dark in the mistiness of the night. I chose a dark night like theme for Oaks.
* Now what you do is think of at LEAST 10 places you could find within your theme, this is CRUCIAL for your layout. For Oaks there was several like Tunnel of love, Entrance, Admin block, Fire exits, maintainance buildings, roller coaster, petting zoos, bouncy castle, slides, carousels, toilets, food court, information centre(the list goes on). The reason this is important before designing your layout is because all the HOT maps are recognizable by every square foot and you need to think of these areas which will be inside your layout before you start designing how they will look inside your layout, eg with TP you have pillars, street, window, toilets, office, back office, station, forklift, pedovan, sign. and the same with Casa.
* Once you've thought of a good list which your happy with then you need to follow what this article reads. http://www.twisted-strand.com/ut_tutori ... index.html
the main bit to follow is the rule of 3. when you spawn you want to split the team up by 3, and each atrium should have at least 3 entrances/exits.
* Now is the part where the real magic happens, What I did is every night before I went to bed I drew a layout. I did all of what I said above with different themes EVERY night for a whole month. I now have about 17 layouts lying around which are worthy of playing. at the beginning you might be making pretty crap ones, but by the end of the 30 days I GUARANTEE you that you will have at least 1 flawless layout, this is one which I designed at least 10 days before the FTWgl competition was announced http://www.gotdelirium.com/site/materials/5.jpg
this layout has now gone on to become Oaks and one of my greatest maps yet. It has been welcomed by SO many people that it could be called my best one so far (but stats don't agree just yet)
Anyway, that's my process.