To charge your chakra stones after you have cleansed them, you have to fill them with energy from a natural source. For this purpose, the best method is to use the power of elemental sources - sunlight, moonlight or earth energy.
Different crystals have affection towards different energy sources.
Some stones like the energy of the Sun (especially stones which work on solar plexus chakra, yellow and orange crystals and quartz crystals). Beware not to expose your crystals to the scorching sun. Like plants, humans or animals, also crystals don't like to be burnt by the Sun.
Other gemstones have the affinity towards the energy of the Earth (grounding stones like Moqui marbles, red jasper, hematite, and other stones which work on the lower chakras.)
Gemstones which work on deeper energy levels prefer to be charged with the energy of the Moon (like moonstone, moldavite, milky quartz and other stones with softer and more feminine energy qualities).
Aside from specific exceptions, you can use any elemental energy source to energize any gemstone - but the effect of the energy which you use to charge it with will give a slightly different feel to the crystal when you use it.