1994 Other Mazda Models overheating

Hi I recently flushed coolant and replced with new good quality engine coolant and 4 weeks later my car has started overheating again, temp gauge on my dash is almost at hot, smoke coming from under bonet and its hissing(sounds like fireworks) Im semi mechaniclay minded but am freaking out with what might be wrong. I have to put water into radiater and the overflow tank to keep temp down. can you help me diaognose the problem? I would like to try fix myself to save money!
Thank you.