To write instructions, start the sentence with a verb. Sometimes that’s all you need, for example:
These are sentences! But sometimes you’ll need to add an object (to give the hearer more information), for example:
Listen to me.
Look at this photo.
Take two slices of bread.
For a negative, just add ‘do not’ or ‘don’t’ to the start:
Do not feed the ducks.
If there are two people listening and you want to talk to only one of them, you can add their name:
Chris, come with me, please.
It’s good to add ‘please’ if you’re speaking directly to someone and want them to do something. This is called a request. There’s more about requests here.