The best way to water your garden efficiently during a dry spell that I always recommend to my clients is by watering it at the right time. If watered during the afternoon, the harsh sun will likely cause all the moisture to evaporate before it has a chance to soak into the soil, leaving your plants thirsty. Instead, water your plants in the morning or evening. Since the sun isn't directly overhead then, the temperature is cooler, allowing your plants to get the water they need while allowing you to use less of it.
For the watering itself, I recommend investing in a drip irrigation system. A drip irrigation system delivers water directly to your plants' root zone, minimizing evaporation. On top of this, the system can be fully automated, meaning that you can still water your plants at the right time, even if you're on vacation or away on a business trip. However, since drip irrigation systems can be expensive to set up, you can also use a hose or a watering can (I only recommend this for a smaller garden) instead.
I would also recommend applying a layer of mulch around your plants as it will help the soil retain moisture and regulate its temperature. On top of this, it will also suppress any weeds, which is always a bonus for any gardener!
Finally, make sure to practice deep, infrequent watering all year round as this encourages deeper root growth, allowing your plants to better handle periods of drought.
