1. Download any set of text-based instructions
The instructions are available here.
2. Have a sighted friend help sort the pieces
Though the following sorting instructions and tips are pretty obvious to the average sighted person, I’m putting them here, just in case.
In most larger sets, the plastic bags with the pieces in them will be numbered. Each bag has sub-bags, e.g Bag 1 has six sub-bags. Take a lot of Tupperware containers or bowls, and dump out all the pieces from bag 1. Then sort them by color. The smallest pieces come in a separate sub-bag of bag 1, so don’t mix them with the other pieces.
Then take a bunch of Ziploc bags, a permanent marker, Lego’s graphical instructions, and the textual instructions, (make sure that the pieces you are putting into a ziploc are the same in the graphical and text-based instructions), and then sort the pieces according to Lego’s instructions. Take another ziploc and label it Extras. Put all the pieces you end up with at the end of the sorting process into this bag, and trust me, you’ll have some extras, with no idea as to where they came from.
Sorting Lego is like taking apart a watch, you’ll always end up with extras, but don’t worry, they’ll find their place during the building process. Next, if the same type of piece comes up in different colors, e.g 1 blue 1×2 amongst grey 1x2s, then put the blue 1×2 into a separate bag, which you should put inside ziploc 1.
Check with the text-based instructions to make sure you are sorting according to their specifications as well as Lego’s. Make sure not to overstuff a ziploc, put 5-20 pieces, per bag; more than that will be unwieldy. If you have 1×1 flat-smooth pieces, in blue, and green, as well as other pieces which are needed for construction of a step, label one bag, A, and the other bag b, put the pieces into a and b as the graphical and text instructions dictate, and put them inside of Bag 1. After you’ve sorted about 20 bags, put them into a large Ziploc bag, and label it Large Bag 1. It’s easier for a blind person to rifle through 20 bags, rather than 50 to find the bag they need. Sometimes the point at which to start a large bag, isn’t the same as in Lego’s instructions, but that’s fine.
Continue sorting until all of the pieces have found a home. Then present the set to your builder and let the building begin!