I was wondering how do people say "lay on one side" in Spanish. I need it as an imperative to use at gym or something(not while you're sleeping). I searched "tumbarse a un lado" but couldn't find any specific result.
You can use different approaches:
Tumbarse de lado
Tumbarse del lado derecho / izquierdo
Acostarse de lado
Acostarse del lado derecho / izquierdo
Since "acostarse" has the connotation of laying for a long time, you can also use the verb "recostar", which means Acostarse durante un breve período de tiempo - laying for a short period which is more related to what you would do in a gym.
As an imperative it would be:
Túmbate (tú) / túmbese (usted) / tumbaos (vosotros) ...
Acuéstate / acuéstese / acostaos ...
Recuéstate / recuéstese / recostaos ...