The approach begins with the Bellevue cable car and then a shuffle over to the Tramway due Mont Blanc railway line, where you put skins on. From there you head up to the Col de Mont Lachat, which you can't really miss as it is right next to a huge old lift station and furthermore there is a signpost telling you where you are. It isn't immensely obvious from there but basically you need to cross the big bowl above and exit onto the left ridge by whichever route looks safest and easiest. I've put a photo below which hopefully makes it a bit clearer.
Our route across the big bowl, taken from the Col de Mont Lachat
Skinning up the Tramway du Mont Blanc