There are various ways to give a command or an instruction. Familiar commands are the most common as they use the Tú  singular) and Vosotros (plural). 

In order to form the positive vosotros command begin with the Infinitive of the verb.

AR verbs

e.g. hablar.  Remove the R leaving – HABLA + D  – HABLAD

ER verbs

e.g. comer.  Remove the R leaving – COME + D – COMED

IR verbs

e.g. vivir.  Remove the R  leaving  – VIVI + D – VIVID

In order to form the negative Vosotros command begin with the all of you or vosotros part of the verb.

AR verbs  e.g. habláis.   AR verbs will take the ER ending ÉIS  – HABLÉIS

ER verbs  e.g. coméis.  ER verbs will take AR verb endings ÁIS – COMÁIS

IR verbs e.g. vivís..  IR verbs will take the AR endings ÁIS – VIVÁIS