The back of the flares have clip holders and of course the red, blue and yellow colored push-in clips that mates the flares onto the body of the truck, problem is is that some of the clip holders will have gaps between it's mounting surface and the trucks panel when mounted. That's due to the factory location of the holes being in different depths.
Using double-sided tape will work on some push-in clip mounts and some you will have to build a layer of to make up for the space in between. Plus the weight of the flares compared to the SR5 factory slim flares are significant.
My suggestion to you is to use some RTV sealant like a gasket maker, it has the ability to stick two things together very well, has a thick composition that can compensate for space between two objects. Also with the double-sided tape you have to be correct in mounting the flares the first time where as the RTV sealant is more forgiving and just as strong if not stronger. Just use an autobody tape to hold the flare pieces in place while waiting for the sealant to dry up.
Good luck!