Forehead lines or wrinkles run from side to side on the forehead. Forehead lines can involve the whole forehead or be worse in certain parts of the forehead.
Often the lateral (outside) part of the forehead is involved in isolation. During ageing the brow starts to sag. The forehead then tries to compensate for the saggy brow by contracting to hold the brow up. This contraction of the forehead muscle causes wrinkles on the sides of the forehead.

Frontalis muscle. This is the big muscle, highlighted in the image, that covers the forehead. The Frontalis muscle accentuates horizontal forehead lines when it contracts.

The Orbicularis Oculi muscle is the ring shaped muscle around the eye. When the Orbicularis oculi is injected with Toxin at the spot marked with the blue question mark in the image below, the outside edge of the eyebrow will lift a bit. This is known as a lateral (outside) eyebrow lift. A lateral eyebrow lift is often combined with Botox injections of the brow and forehead.

The patient sits back on a bed with the backrest up.
The area is cleaned.
The exact placing of the injection points are different between people and depends on the muscle action observed when wrinkling the forehead.
Botox injection of the horizontal forehead lines is always combined with Botox injections of the brow, because injecting the forehead without the brow might lead to heaviness or even slight sagging of the eyebrows.
The muscles are injected.
It takes about 1-2 minutes.
Light pressure is applied to the area to prevent bruising.

Procerus muscle injection (highlighted in the image above) for forehead lines:

Orbicularis oculi muscle injection, marked with the blue question mark in the image above, for a lateral eyebrow lift:

Do not go to sleep or massage the forehead, because this might displace the Toxin toward the upper eyelids, leading to a heavy feeling of the eyelid or even a slightly droopy eyelid. Should this happen, Iopidine eyedrops will counteract the effect until it resolves spontaneously in 7-21 days.
Some patients experience a slight headache that can be relieved with Paracetamol.
Slight bruising can occur, but can be concealed with make-up.
It resolves in a few days.
The effect will kick in after 7-10 days and last between 2 and 7 months, depending on the dosage used and the patients physiology.
Some patients dissolve Toxin faster than other patients.
First time patients will sometimes be requested to return after 2-3 weeks for a check-up and top-up if required.