There's a couple of ways you could do it. You could join the spreadshirt affiliate program and then add a link to their "Spreadshirt Designer" on your site.
You could also add a product to your spreadshirt store, select the TEXT option and then select the "allow your customers to change the text" option.
You can also have their custom t-shirt designer tool branded for your business for a fee. More info on that here:
http://www.spreadshirt.com/Earn_Money.401.0.html
You could also add a product to your spreadshirt store, select the TEXT option and then select the "allow your customers to change the text" option.
You can also have their custom t-shirt designer tool branded for your business for a fee. More info on that here:
http://www.spreadshirt.com/Earn_Money.401.0.html