How can I create a mob spawn like golem's spawn method

Depends on what blocks you want to use to make the golem. If its custom blocks, or at least one custom block. Make the custom block a block entity and make it tick. Then on block update (example of making iron golem) check for block at x y-1 z = iron block, block at x-1 y-1 z = iron block, block at x+1 y-1 z = iron block, block at x y-2 z = iron block, if all = true then remove blocks at said locations and spawn entity (iron golem). You would also need to check which way the block is facing. Eg if it is facing north or south, which is the z axis. Then the arm blocks will be at x-1 and x+1, whereas if the block is facing the east or west, which is the x axis, then the arm blocks will be at z-1 z+1. May have gotten those axis mixed up cant remember. So you would make two sets of procedures depending on which axis the golem is being made on. That is if the shape has arms, if its a snow golem looking one then you dont need that and you only need to check for block at x y-1 z and x y-2 z.