La farmacéutica me explicó cómo usar el hilo dental sin lastimarme.

In everyday Latin American Spanish, la farmacéutica most commonly means the (female) pharmacist (or sometimes a female pharmacy professional).

• El farmacéutico / la farmacéutica = pharmacist (male/female)
• La farmacéutica can also mean the pharmaceutical company/industry in some contexts, but in a sentence like this (someone explained something to me), it clearly refers to a person.

La is the definite article (the), used when the speaker assumes the listener can identify which person it is (e.g., the pharmacist who helped them).
Una farmacéutica would be a pharmacist, introducing her as someone not already identified or not important which one.

Me is an indirect object pronoun meaning to me.
Explicar typically takes:

• who receives the explanation (indirect object): me/te/le/nos/les
• plus what is explained: a noun or a clause

So: (Ella) me explicó (algo) = She explained (something) to me.

Explicó is preterite (simple past): she explained / he explained / you (formal) explained.
The accent shows the stress and also helps distinguish it from explico (present tense: I explain).

Conjugation contrast:

• yo explico = I explain
• ella/él/usted explicó = she/he/you explained

No—explicó cannot mean I explained. It’s third person singular (or usted). The subject is la farmacéutica.
If it were I explained, it would be expliqué.

Cómo with an accent is an interrogative/exclamatory word meaning how (direct or indirect questions).
Here it introduces an indirect question: she explained how to use...

Without the accent, como usually means as/like or I eat (from comer, depending on context).

They’re very similar, but slightly different in feel:

• me explicó cómo usar el hilo dental = she explained how to use floss (more direct, instructing you)
• me explicó cómo se usa el hilo dental = she explained how floss is used (more general/impersonal)

Both are natural in Latin America.

After cómo, Spanish often uses an infinitive to express how to do something:

• cómo usar = how to use
  Using uso would change the meaning and structure (it would sound like how I use and would need a different construction).

El hilo dental is the standard term for dental floss.
Hilo alone just means thread (general). Adding dental specifies it’s for teeth.

Lastimar = to hurt/injure (someone).
Lastimarme includes me attached to the infinitive, meaning hurt myself.

So:

• sin lastimar = without hurting (someone/anything) — incomplete unless context supplies the object
• sin lastimarme = without hurting myself (clear and specific)

Both are correct:

• sin lastimarme = without hurting myself (focus on avoiding injury as a condition)
• para no lastimarme = so as not to hurt myself (focus on purpose/goal)

In practical use, they’re often interchangeable here.

With an infinitive, Spanish allows pronouns to be:

• attached to the end: lastimarme
• or placed before a conjugated verb (if there is one): e.g., no quiero lastimarme / no me quiero lastimar

Here there’s only an infinitive after sin, so attaching is the normal choice: sin lastimarme.