At a time

At a time
 

IS THIS SENTENCE correct?
Go to discussions and make sentences using the following expressions the ones you feel more comfortable about , but use only one at a time.

Without more context, I have to assume that your audience will understand what 'go to discussions' means. As it stands, it doesn't make sense to me. In order for me to understand it, you'd need to include more specifics, like 'gather/separate yourselves into discussion groups,' or 'go to the section of the assignment labeled 'Discussions,' or 'go to another area in the room/building where discussions are held.'

After the clause 'using the following expressions,' there should be a comma (a comma doesn't have a space before it).

The last clause you wrote is correct, and can only be a reference to how many expressions should be used per sentence created.

or 'go to the section of the assignment labeled 'Discussions,

there is a section labelled discussions