Align your object with the object to be replaced, taking several shots with an increasing awareness of how the two will fit together
You must shoot the same day, at the same time, with the same lighting. otherwise you will know something is wrong with the photos, but you won't be sure what it is. What's wrong is that the shadows go different directions. Also two different light sources, like a sunny day and cloudy day, will look weird.
Tell some kids they can eat cookie dough when we're done and also have their picture on your web page.
Take several shots and take the best selections out of those shots
Now find a friend to Photoshop the pieces together!
Take many more photos that you think might work out for something. Of course they never will work out, so you can put them at the end as an out-take, like this.