(someone) is supposed to (do something)

Say that someone "is supposed to" do something if:

• there's a rule about it
  

  I think we're supposed to shower before going in the pool.

• there's a custom that says something about it
  

  You're supposed to take your shoes off when you go into someone's house in Japan.

• you have already agreed to do it
  

  I'm supposed to pick Janelle up from school this afternoon. 

Note that "supposed to" can also be used in other situations, like when you're reporting information that you heard from other people.

It says to (do something)

When you read some instructions and then report them to another person, use the phrase "It says to ___." For example:

It says to add the onions and celery and simmer for five minutes.

For warnings, say "It says not to ___":

It says not to touch the connectors.

You can also specify where the instructions came from:

The sign says not to go in there.

It might seem strange to use "says" with written instructions since the instructions don't actually speak. But English doesn't have a separate verb to describe information that's written down, so we use "says".

use (a chemical) in a well-ventilated area

"Ventiation" means allowing air to move between the indoors and outdoors. This can be done with windows, vents, or fans.

Chemicals which are dangerous for people to breathe often have a warning. The warning says that you should use the chemical in a "well-ventilated area", so that the person using it can get fresh air.