As your new lawn becomes established with young seedlings you can, depending on the weather, ease on the watering. When you were waiting for the seeds to germinate you were probably watering every day but that would be too much now and could rot the young seedlings and stop root growth.
Try missing a day of watering and if the grass seedlings don’t dry out stretch that even further till you are only watering once or twice a week.
Using a sprinkler is the best way as it waters evenly and to avoid damage try not to walk on your new lawn at this stage. When the grass gets to 2-3” high it’s ready for its first cut but it’s a good idea to re-firm the soil with a roller first and then cut it a few days later. Make the first cut quite a high one taking off only a third of the growth and if your lawn was autumn sowed you won’t need to cut again till the following spring. A spring lawn will have to be cut again about every 5 -7 days reducing the height of the cut each time till you reach a normal cut level. Frequent cutting will encourage a denser lawn with fewer weeds.
You might want to give your lawn a boost after 4 weeks with a lawn fertiliser and then feed it every spring and autumn. Once established you can use products like Westland Aftercut if you feel it needs greening up. An established lawn can survive with no watering so it then becomes a personal preference. If you want a lush green lawn all year then you’ll want to water in the summer but if you don’t mind it going brown in very hot or dry weather then just leave it to look after itself. A well-established lawn with a good root system will always bounce back.