Spamming cubes will give you random layouts, which will most likely not provide the best gameplay. Just putting individual segments together mostly also doesnt work out, because the layout has to work as a whole thing. Drawing your layout may help you, if you find it hard to imagine the whole thing in your head, but won't actuall create a layout. I often draw parts of my map, but usually for esthetic reasons, not the layout.
I would suggest you start of with some idea of a gameplay situtation or arrangement of rooms you want to have. Just something that will for sure make your map special, not just yet another dm layout. It could be anything from a tube with a jumppad at the botton and a quad at the top to an unique teleport setup or whatever you can think of. Once you have some idea try to imagine things around it until you have a full layout in mind, not everything has to be connected yet, but every segment has to add smething to the gameplay. Then block your layout and fill all the places that were missing in the layout you imagined. When you for example have too rooms on a different height level, dont just connect them with a ramp and rush to the next place, but think if a curved stairs, a jumppad or maybe even a teleport made more sense there.
All this may sound pretty complex, but after thinking about some ida for maybe a week or even less i know what i gotta do and can usually get it roughly blocked in sauer in less than 15 minutes. This step consumes the lowest ammount of time, but is extremely important, because it'll be you guideline for the rest of the building process of your level. However, it's of course still FAR from perfect and this is the time when playtesting comes in. Quickly spam some lights around so players can see shit, pay a bit more attention on the playerstarts and if you want add pickups already and then PLAY PLAY PLAY. Find someone who is somewhat good at the game, but more importantly knows how to make proper maps, to duel with. try to find out how well the map works for yourself and listen to your opponent, what he thinks is good or bad. In this early stage its very important you that havent started detailing anything, so you wont be afraid of losing your work when redoing things that didnt work out. Try to test the map with as many different people as possible and as often as possible. Change whatever you think is not pefect and if you think its good, change random things and see if it gets even better, if not, of course change them back again.
Only then when you are sure your layout is final (it most likely isnt, but its the clostest you will get until starting with the details
), start detailing. That will reduce the ammount of lost work a lot, since you hopefully only have minor changes left to do.
Well, this is just how do my stuff. There are many other ways, but i hope ive at least given you some ideas.
