3. Put the tag somewhere on your agUF frames, e.g. select "Custom" as class text in the player frame and put [druidmana] in there
Note, that the bar from SimpleDruidBar will be shown, so you might want to resize it to 0x0 with alpha 0.0 or move it offscreen.
Also you need to enable "Show enhanced tooltips" in your interface options, as SimpleDruidBar needs them to determine the mana cost of the shape shift spells. Alternatively change line 125 in SimpleDruidBar_Display.lua from
- (SimpleDruidBar_ParsingTooltipTextLeft2:GetText():match("(%d+)") or 0)
-- (SimpleDruidBar_ParsingTooltipTextLeft2:GetText():match("(%d+)") or 0)
(this will make the estimation inaccurate, due to ignoring the mana spent for shapeshifting)