Same as normal. The Mixin that is applied when using the template applies all the pre 9x backdrop methods to the frame. Nothing really changes once you've inherited the template.
Lua Code:
if not f.Backdrop then
f.Backdrop = CreateFrame("Frame", name.."Backdrop", f, "BackdropTemplate")
f.Backdrop:SetAllPoints()
f.Backdrop.backdropInfo = {
bgFile = "Interface\\DialogFrame\\UI-DialogBox-Background-Dark",
edgeFile = "Interface\\DialogFrame\\UI-DialogBox-Border",
tile = true,
tileSize = 32,
edgeSize = 32,
insets = { left = 11, right = 12, top = 12, bottom = 9, },
}
f.Backdrop:SetBackdrop(f.Backdrop.backdropInfo)
f.Backdrop:SetBackdropColor(r, g, b, a)
f.Backdrop:SetBackdropBorderColor(r, g, b, a)
end
f could just have easily inherited the backdrop template negating the need for a separate frame unless the backdrop is applied after the frame is created eg. something like KGPanels where a backdrop can be added/removed at runtime.
Example