How walls and obstacles can be created in matlab with using some shapes or sqaures (grid map)?

I am doing SLAM, I have been told to create map for robot which will see it and if any obstacle is found it will avoid it and at the same time will maintain its position to reach goal. I know MATLAB has SLAM toolbox but I cannot used I need to create my own algorithm. I don not need code just need guidance ,like how to proceed further.