Here are few advices on how to straighten your hair in a professional way:

Advice 1: Pick the correct size hair straightener

The size of the iron should depend on thickness and length of your hair. Using the right size iron will allow you straighten your hair more effortlessly and reduce the risk of damaging and overheating hair.

Straighteners with narrow plates ranging in size from half-an-inch to one-inch thick are used for short, fine or thick hair. The best way to iron your hair is dividing them in smaller sections rather than big.
For long hair: Use an iron with wider plates ranging in size from 1 1/2-inches to 2-inches wide so you're able to cover more surface area in less time.

Advice 2: Work with the correct material

Seek for hair straighteners that use ceramic, tourmaline or titanium metals. They will heat on an even plane causing less damage to your hair. Irons made from these materials release negative ions which work to the hair's cuticle. This positively effectuates your hair making it healthier, shinier and less frizzy. Avoid straighteners which are just coated in a ceramic lacquer that wears off over time and can expose your hair to bare aluminium.

Advice 3: Choose the correct heat setting

Selecting your heat setting you should pay attention to your hair type and texture. The advantage of your hair straightener would be a variable heat setting. When you have a choice to select the right temperature that’s suits your hair, the less likely you will be to damage the hair.
For fine or damaged hair: 180 - 200 degrees
For medium hair: 210 degrees
For thick or coarse hair: 235 degrees

Advice 4: Get ready your hair properly

When the best straightener is selected, the next important step in achieving the results you want is getting ready your hair for the heat.

Getting ready the fine hair: Wash hair with a strengthening volumizing shampoo like AMETHYSTE Volume Shampoo and Conditioner, both by FarmaVita. Afterwards use a volumizing mousse like FarmaVita HD LIFE STYLE - Volume & shine mousse.
Blow dry hair with a round brush, lifting at the roots to add volume. 

Getting ready the thick/course hair: Wash your hair with a FarmaVita Linea Vita smoothing shampoo and mask. Next apply a smoothing serum heat defence spray by FarmaVita. Blow dry hair thoroughly with a natural bristle brush and try to make it as straight as possible.

Advice 5: Achieve a super straight look

In order to get a super straight, polished look, follow these few steps:

Step 1: Blow dry your hair with a paddle brush and divide it into workable sections. Start with the lower layers.
Step 2: Smooth the iron down the hair shaft, starting half-an-inch down from the roots to maintain some volume.
Step 3: First straighten the bottom and mid-sections of your head.
Step 4: Then get to the sections near the crown of your head, try to place the iron as close to your roots as possible, pressing tightly while moving the iron straight towards the ends of your hair to accomplish a super smooth finish. 
Step 5: Use the shine spray like HD LIFE STYLE - Vitamin booster extra shine spray to finish the look.

Our recommendations: 

Never straighten your hair when wet, because you might cause severe damage to your hair. Always blow-dry your hair prior to straightening.
Do not use a hair straightener everyday as all procedures involving high heat will dry the hair over the time, especially with frequent use.