Equip the Warrior with the Stone and then in a battle or a training session, use it on that Pokemon. And voila.
in battle go to warrior and there should be a fire stone there and use it and after battle he should evolve
To use any of the stones in Pokemon Conquest, you have to equip the pokemon compatible with the stone, go into battle (preferably a training battle and not one to conquer a castle, but that's just me), if you win, you should get a scene of the pokemon's owner says something along the lines of "Something is happening to [insert pokemon here]," then the little evolution scene will happen. The only ones I've done are Thunder Stone for Eevee to Jolteon, Water Stone for second Eevee to Vaporeon, and Moon Stone for Jigglypuff to Wigglytuff.
you can use a water stone on lombre
Sunkern and Gloom use a sun stone Lombre, Staryu, and Eevee use water stones.
you use it, assuming you have a Pokemon that requires a water stone to evolve instead of just simply leveling.
Lombre and etc
you use it on some water type Pokemon to evolve them. example:pikachu+thunder stone example:polywhirl+water stone example:evee+fire stone example:Roselia+sun Stone example:murkrow+dusk stone a ever stone prevents your Pokemon from evolving
There is only one pokemon you can use it on and that is eevee to get vaporeon
Staryu.
an Eevee, use a water stone on it to evolve it into a vaporeonEevee into VaporeonPoliwhirl into PoliwrathLombre into LudicoloShellder into CloysterStaryu into Starmie
get an eevee and use a water stone on it. Simple!
Use a water stone on poliwhirl