how to add flowers ?

The details are known as detailsprites and are tied to specific materials. You can see if it is a detailsprite-ready material if it has some small shapes on the bottom right of the texture preview in hammer(can't get a screenshot right now, sorry). You also need to specify the detail .VBSP file in the Map Properties window, look at existing maps with the texture to see what they use. Just note that the sprites do not appear in hammer, only within TF2 for whatever reason (they used to work in hammer but that broke)

Note that the supposed shapes is an optional part of the ABS pack, that is not a standard feature.