This is probably really simple but I keep getting confused by which form I’m meant to use. Do I match it to the person I’m talking about or do I match it to the gender of the noun? And which part of the sentence do I use to determine whether I use the singular or plural?

For example “Le mando notas” means I send him notes. What about a singular note? If I’m sending her notes does it become la? If I was sending them notes would it be las or les? If notas is feminine why don’t I use la?

Is there a simple way to remember whatever the rule is? Thanks!