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Tips are hard to give, but I'll give it a shot. Basically you need to know what you want to use the map for. It's hard to create a good all-round, good for everything map. From there you should just try to keep an even amount of cover and high-ground for each half of the map. Don't worry so much about aesthetics, they can wait until the end. And don't make massive, complicated structures, oceans of grey look bad, and use too much of the budget.