Voi and noi forms in the positive -
Attach the reflexive pronoun to the end of the conjugated verb
E.g
Lavatevi subito! - Wash yourselves straight away!
Svegliatevi presto! - Wake yourselves up early!
Sediamoci qui! - Let's sit down here!
Alziamoci presto! - Let's wake ourselves up early!

Noi and Voi forms in the negative -
The reflexive pronoun is attached to the end of the conjugated verb, the infinite is NOT used for the noi and voi forms.
E.g
Non svegliatevi presto! - Don't wake yourselves up early!
Non lavatevi subito! - Don't wash yourselves strait away!
Non sediamoci qui! - Let's not sit down here!
Non alziamoci presto! - et's not wake ourselves up early!