I quite like how phonearena.com handles this.
You have 2 options to compare items:
In the left sidebar, there is a link called "Compare phones". Click it and it will send you to a page where you can search for an item. Hovering over an item will reveal a "Compare" button. This could be done a little bit better, so instead I would show the compare button on each product tile.
Pressing the compare button will add the product to the comparison queue, where the final Compare button lights up green when you have selected 2 or 3 products
Go to the specifications page of a product, there, just above the specifications, you will see a compare button. A page will be loaded with the specifications of the product, and 2 white columns with a textbox above it and the text "Add phone to compare". In the textbox you can search for a phone and while typing, it will show a list of products. Choosing one will add all specifications for that product in the blank column.
When in the comparison, you can delete a product by clicking on the X button, or you can add a product by typing in its name.
Something else I really like what they did is give visual cues as to how big/strong/fast everything is, by including 3 icons, where one is blue and 2 are grey. The one that is blue shows whether the phone is in the top of its class in that department, the middle or the bottom. One problem with this is (espacially with smartphones) is how they evolved over time, whereas 233 pixels per inch was ultra-high end back in the day, now it's comparatively low, so in thos cases some extra work has to be done, like comparing the spec against the average of all products with that spec or some other calculation. Depending on the subject you are covering, this doesn't have to be a problem though, as for example fruit will have (almost) the same nutrition values for decades.