Hi Nikita, Simple same as we are doing normally only variant tab like things not there but when you select a product in bom then you will find it will reflect both product main one and the variant one. 

For better Understanding you can give internal reference number of your variant goods then you will find at the time of selecting product as bom your variant is reflecting. Apart of it if you need any help then you can ping me. Or mail me business@erpcall.com