Add to Cart and Checkout process

I have created a customised product that returns a few options of results to the product.tpl in a table form. I need to select an option and proceed to checkout. At this time, I am happy to follow the standard checkout process with a view to modifying it at a later stage.
What I would like to try and find, is the process from the product through to the checkout process.
What I have managed to find, is that two variables should be passed, namely product_id and quantity, but am battling to fathom how they are passed and then interpreted. So, I have created a simple form to test this that submits these two variables, but it fails with the following report:-
{"error":{"option":{"226":"Select required!"}},"redirect":"http:\/\/localhost\/index.php?route=product\/product&product_id=30"}

Form sample:-
<form action="index.php?route=checkout/cart/add" method="post" enctype="multipart/form-data" class="form-horizontal">
Product id:<br>
<input type="text" name="product_id" value="product_id">
<br>
Quantity:<br>
<input type="text" name="quantity" value="quantity">
<br><br>
<input type="submit" value="Submit">
</form>
So, is anyone aware of any documentation, or an understanding that I may read to try and work the checkout process out please...

From product page if you add something to cart the variables are stored in cart class best way to track how it is all working is to track hos options is made:) for product