First of all you will need to have created custom field in Blacthorne which will describe your product like size,colour etc.
So for example you are running shop with t-shirts so we talking about colour and size in this example.
In template editor under Ad template Studio in Item Fields (left column) are all of fields listed included your custom.
Let say you have custom field Size and Colour created in Blackthorne for sizes of your product, I will explain based on html code:
<P align=center><A href="http://stores.ebay.co.uk/in here your store name/_i.html?_nkw=[[Size]]+[[Colour]]+&submit=Search&_sid=your sid number goes here"><FONT color=red>your text goes here for example more T-shirts in [[Colour]] and in [[Size]]</FONT></A>
Remember to ad store name and your sid correctly.
Easy like that, you could include drop down if you are more familliar with html.
Basically that is it.
I have used at http://stores.ebay.co.uk/rimandtyredirect look at OTHER WHEELS FOR:.....