To offer a buy one get one free scheme on your store, please follow the below steps.
1) Decide on which product you would like to offer this particular scheme for.
Note: this can be done only for products that are of the same value.
2) Create discount coupons for that particular amount from the Zepo dashboard.
3) You can display this offer on the store with banners or also include this in the product description. Now you can expose this discount coupon code and let customers use this code and add 2 products to the cart, but end up paying only for one.
Please find this example useful in understanding how it can be done.