What are the basic tricks to prevent the bottom layer of cake from burn when making it without an oven?
I am guessing from your question that you are making a stovetop cake.  

Burning the bottom of the cake is probably related to too much heat transfer directly to the cake pan.  Most recipes that I have seen incorporate some method to separate the pan from the direct heat. 

If you are making the cake with the assistance of a large pot or dutch oven, then it is important to make a rack to keep the cake pan off the bottom of the outer pot. The most common way of doing this is by creating a crunched up ring of aluminum foil, and placing the cake pan on top of that foil ring. You could also use an upside colander.  I've also seen techniques that use cleaned gravel or pebbles to keep the cake pan off the direct heat. (For example, check out this website: https://www.nigerianfoodtv.com/how-to-bake-cake-without-oven-and-on/)

If you are cooking directly on the stovetop (without using a dutch oven), then you probably need to decrease your heat and go for slower cooking. Lining your cake pan with foil or parchment may also help. 

Good luck!