Very easy...
Create a marquee.
Select the layer you wish to align TO it.
Select the 'Move' tool.
Use the vast array of 'Align' tools presented in the menu bar at the top of the UI...
View attachment 75038
You can see from this that you just need the marquee to go FROM the left of the image TO the right.....height is irrelevant if you just want to align horizontally.
If the layer contains ONLY the pixels with-in the shirt you can just CTRL click the layer thumbnail to get a really quick 'boundary' marquee....then align to it.
In a nutshell, if you have TWO layers selected you can align them to each other....but without a marquee they average out their alignment....because PS doesn't know, and you cannot tell it either, which one should take priority.
OTOH, If you have a marquee present when you click an alignment tool then everything is aligned relative to the marquee.
For example to align to the document use 'Select All'...'CTRL + A'...then align.
'Select All' creates a marquee along the document edge.
Lots of ways really....and really, really, handy.
For best results, always have a document of EVEN pixels dimensions, and always have EVEN marquee dimensions....this way you can divide by two without trying to split the pixel...which you cannot do.
Regards.
MrToM.
