Pastel hair is so beautiful, isn't it? I'm currently rocking mermaid/unicorn hair - and let me tell you: It's a process and a lot of work!
First of all, you need a blank canvas. That means the hair has to be devoid of all color. Getting the hair to white/silver is damaging and may cost you quite a bit of length in the end. You can have lovely cotton candy locks or you have long, flowing locks - but you probably can't have both.
I am a huge, huge, huge proponent of taking things slow. You should never sit in a stylist's chair as a brunette and request platinum or pastel hair in one sitting. Hair lifts in steps and the middle step is usually orange. And that's ok! Personally, I chose to embrace the orange and go whole hog with it. Did I look like a modern-day reincarnation of Vitamin C? Yes! Did my boyfriend hate it? Totally. But he doesn't understand chemical damage to hair and I do, so he had to live with my fiery tresses for a bit. (He was like bull, apparently. Seeing red made him *angry*)
Besides getting there cautiously, I think the one thing I want people to know is that it's expensive! The bleaching plus product can cost anywhere from $200-$700 and up to 5-6 hours in the chair. Your whole day AND your whole paycheck.
The other important thing to keep in mind is having product on hand at home. Here's why: Pastel colors only last 1-2 washes. Seriously. You will not be an exception. And so you will put off washing your hair as long as you can. Dry shampoo and shower caps will become your best friend. You should -although I've never been able to pull it off - wash and conditioner your hair only with cool-to-cold water, as hot water strips color. Right now, to maintain my soft pink hair shade, I have a tub of Manic Pan Fleur de Mal, a tube of Pravana Vivids Pastel Pink and a tube of One 'N' Only Argan Oil Bubblegum Pink pastel hair color, as well as Davine's Alchemic Red conditioner and a Jerome Russell Splat Pink-augmented bottle of shampoo. Yes, it takes an array of products to be a unicorn. Deposit new color before every wash, or risk starting to rock "granny hair'. Honestly, dry shampoo also helps hide my dark roots in addition to soaking up oil between washes.
My last pastel hair care tip is to commit to styling, shine products and Olaplex treatments. Your hair will not be as reflective or full or luster-y as when you were darker and your hair was less compromised. Use spray shine and serum when hair is wet. Use low-level heat curling irons to style for work to look more polished or use rollers and wet-styling for no heat at all when you are this light
When you are initially lifting, I would not proceed bleaching with Olaplex. Olaplex is a patented treatment - a "bond multiplier" that mitigates damage in the lifting process. It's cruelty-free and there's nothing like it. There's three parts. Step 1 is added into the bleach and actually slows down the developer a bit. Step 2 is added as a leave-in treatment after dyeing and Step 3 is generally used as a take-home conditioner. (I leave mine on all night!)