Let them see clear the difference...!
Let your child know from the beginning what day and night means. So when you start the day open the curtains and brighten up the room. Start your day with them as usual. Interact with them so they know it's playtime. You can't do much wrong at daytime.
But when it comes to sleep time at night there are some things you should keep in mind...
* have the last feeding in the dimmed (night light) room.
* no more talking (especially when they wake up at night)
* just do what is necessary to get your child back to sleep (nursing, change diaper, some love..)
What you also should think about is..
Don't let them oversleep during the daytime naps. They need at different ages different amounts of sleep ( of course there is here and exceptions). In general, if you let them nap too long during the day it's most likely they won't sleep at night as you want it.