Bitofabelly, you mix one part Illumina 10/69 colour with two parts developer as explained in the picture. Rinse out and wash and condition as normal.
This is just a toner, so changes the blond to a cooler pastel shade but won't really make it lighter / lift the colour. Your hair needs to be either pre-lightened or be naturally that light (a level 10). If your hair is for example a darker blonde, so a level 8 or 9, you will still see an effect but it won't be quite that colour and will wash out a lot quicker. Basically, what you need to do is determine the level of your base colour and then find the suitable toner, so if you want to make your darker blond cooler, you could use Illumina 8/69 or 9/6.
I have shoulder length hair with dark / mousy blond roots and lighter, pearly blond balayage lengths and ends, so I use 8/69 on top and 10/69 on the lengths (apply at the same time and then comb through hair so the two colours sort of melt into each other). I need about 10-15 ml of the 8/69 and 15-20 ml of the 10/69 for that and freshen up the colour about every for weeks or so (my hair appears to have a lot of red undertones, I wash it every day and live in a very hard water area, so need to this quite frequently to keep any brassiness at bay). That means one 60 ml tube of Illumina lasts me for about 3 months which works out pretty cheap.
Of course other brands have similar products, but I haven’t researched the shades for those.
Disclaimer: I'm not a hairdresser, just someone who was fed up with forking out major €€€ for frequent colour appointments when you can use the same salon products at home with very similar results. I also use Olaplex #1 and #2 (again, the pro product rather than the less potent consumer version) at home to keep hair in good condition.
Sorry for the essay! There is also an AMA thread by a hairdresser you could ask / double-check with. 