If you miss a day or so - no worries, go at your own pace.
Note; if you see liquid on the top (its called houch), that means its fast acting - you can either slow it down or add flour a little more often. To really feed and build, you add the same amount of flour and water to the starter. But you can add a little flour now and then if you just want to give it a snack.
to slow things down put it in a cooler place - to get it more active , get it warmer.
This is the "challenge" of culturing - just getting to "know" it.