Antes de dormir, uso hilo dental y luego me cepillo los dientes.

Because Spanish often drops subject pronouns when they are already clear from the verb ending.

• uso = I use
• me cepillo = I brush

The endings already show first person singular, so yo is not necessary.

You could say Yo uso hilo dental..., but that would usually add emphasis or contrast, like:

• Yo uso hilo dental, pero mi hermano no.

After antes de, Spanish uses de + infinitive when the subject is the same.

So:

• antes de dormir = before sleeping / before going to sleep

You need the de here.
So antes dormir is not correct.

A useful pattern is:

• antes de comer
• antes de salir
• antes de acostarme

If the subject changes, Spanish normally uses antes de que + subjunctive:

• Antes de que te duermas, apaga la luz.

Yes, but the meaning changes slightly.

• dormir = to sleep
• dormirse = to fall asleep
• acostarse = to go to bed

So:

• Antes de dormir = before sleeping / before going to sleep
• Antes de dormirme = before I fall asleep
• Antes de acostarme = before I go to bed

For a bedtime routine, all three can work depending on what you want to emphasize.

• If you mean as part of my bedtime routine, antes de acostarme is very natural.
• If you mean before I actually fall asleep, antes de dormirme fits well.
• Antes de dormir is broad and perfectly understandable.

Because in Spanish, with a verb like usar, the article is often omitted when you mean something in a general sense.

So:

• uso hilo dental = I use dental floss
• uso champú
• uso gafas

This sounds like talking about the item as a type of thing, not a specific object.

You would use el if you meant a specific floss:

• Uso el hilo dental que compré ayer.

Hilo dental means dental floss.

Yes, it is a normal and widely understood term in Spain. You may also hear expressions like:

• usar hilo dental
• pasarse el hilo dental
• limpiarse los dientes con hilo dental

So the sentence is natural and clear.

Yes. Luego and después can both mean then / afterwards here.

So these are both fine:

• uso hilo dental y luego me cepillo los dientes
• uso hilo dental y después me cepillo los dientes

But be careful with después de:

• después = afterwards
• después de = after + noun/infinitive

Examples:

• Después, me cepillo los dientes.
• Después de usar hilo dental, me cepillo los dientes.

Because Spanish commonly uses a reflexive-style structure for daily grooming actions done to yourself.

So you often hear:

• me lavo las manos
• me peino
• me cepillo los dientes

Here, me shows that the action is being done to yourself.

Without me, cepillo los dientes is grammatically possible in some contexts, but it is not the usual everyday way to express I brush my teeth.
The most natural standard phrase is:

• me cepillo los dientes

With body parts, Spanish usually prefers:

• a pronoun like me, te, se
• plus the definite article (el, la, los, las)

So Spanish says:

• me lavo las manos
• me corto el pelo
• me cepillo los dientes

rather than:

• lavo mis manos
• corto mi pelo
• cepillo mis dientes

This is because the pronoun already makes it clear whose body part it is.

Using mis dientes is possible only when you want extra emphasis, contrast, or clarity, but in normal speech me cepillo los dientes is the natural choice.

Because the normal idea is that you brush your teeth as a set, not just one tooth.

So Spanish naturally says:

• me cepillo los dientes

If you were talking about only one tooth, then you would use singular:

• Me duele un diente.
• Me cepillo el diente que me operaron.

But for the everyday activity, los dientes is the standard form.

Because the present tense in Spanish is often used for habits, routines, and things you do regularly.

So:

• Antes de dormir, uso hilo dental y luego me cepillo los dientes.

means something like:

• Before going to sleep, I use dental floss and then brush my teeth.
• As part of my routine before bed, I floss and then brush my teeth.

This is exactly how Spanish normally expresses habits. You do not need a special tense for that.

If you wanted to stress the habitual idea even more, you could say:

• Antes de dormir, suelo usar hilo dental y luego me cepillo los dientes.