Especially for pastel pink you need a very very light hair colour so you will need to bleach your hair if you dont. After that if you're going a cool pastel pink, you need to take out the brassy tones out of your hair with toner. For rose gold or other warm pastels you can skip that step.
If you don't have to bleach your hair or tone it, the rest is super easy to do it yourself. You pick a BRIGHT shade of pink from any vegetable based dye brand such as Adore, Manic Panic, Directions etc. NOT pastel, but a very strong color. The important thing is that the undertone matches what you want to achieve. So if you want a peachy pink you'd go for a bright pink with red/yellow undertones, if you want baby pink you would go for a bright pink with blue undertones etc.
Then you mix in a few drops or squirts of the dye into a huge tub of cheap conditioner or hair mask until you achieve the desired colour. Obviously the more dye = the brighter/darker the colour + the less you have to bleach your hair.
Putting it on when it's diluted in conditioner is just like putting on normal conditioner, very easy. You can do it on wet or dry hair and can leave it as long as you want without it damaging your hair. The longer you leave it the more intense and longer lasting the colour will be. A ton of people leave it overnight, even for pastels. If your dye mix is a light pastel, you're not going to wake up with neon pink hair so don't worry!
I can advise on different dyes based on 15+ years of pastel pink hair ( and various other pinks). But the right dye depends on what you can buy in your area, and what you want your final colour to be. Hair crazy forums have a ton of pictures and info, which I cant link right now but it's easy to google.
Also speaking from experience, a dye you dilute at home will last 5x longer than a pastel dye they use at a hairdresser. I've done pink at a ton of hairdressers in 4 different countries and they all washed out in a few washes.