How do I write a marketing plan for a supermarket business?

Supermarkets are actually very difficult businesses.  They
have small profit margins, but very high product turnover.  Supermarkets basically
survive by selling a lot of product and making a small profit on each item.  Competition
is fierce and the price differential slim.  A marketing plan should focus on getting
customers into the store, and hoping that they will buy more than one
item.

Since your question is a little vague, I would
suggest choosing a product for which the profit margin is a bit higher.  It might be a
product acquired at a larger discount than normal.  Then, you will be able to lower the
price and advertise a lowered price.  Since most people buy more than one item at a
supermarket, you can make a higher profit on other items the person buys but you have
given them the incentive to get into the story.

You can
create a display for the product, and include other items that are similar and have
higher profit margins near it.  Customers who came to buy that one product may also buy
the other.

Otherwise, general supermarket marketing is
fairly typical.  Generate colorful displays of products you want to sell fast.  Discount
items with high profit margins, and place impulse items, such as bakery items, fruit and
junk food, in aisles and next to the checkouts.