At first I was going to use Hobby Glue but I have found it to be hopeless not just with this material but everything else. So I just used regular super glue.
You don't need much of it, just strategically apply it to places that make contact with the surface of the toy. The advantage of Superglue is that it settles quickly so you don't have to hold it for long until it dries. That's the trick.
If you have excess superglue (assuming you didn't apply to much) you can always scratch it off with your own nails when it dries and then wipe it with a wet cloth. It doesn't leave marks, only a white residue which can be wiped.
The trick is to only apply enough glue and hold it in place for at least a minute and then allow it to dry till the next day before transforming.
Important note: The front bumper is fairly fragile so be careful not to break it. The part that I found the hardest by far when it comes to gluing was the rear spoiler mainly due to me being a perfectionist in aligning it right

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The second hardest was the head as I found the Bumblebee head very difficult to remove but it seems that others don't have this issue.