There are no Halo LEGO sets, so there are inheritly no instructions. Mega-Blocks got the rights to Halo (to build children's toys) 5 years ago, and have been dutifully building sets since then.
You can find custom LEGO instructions inside of every pack of LEGOs. There are web sites where you can find custom LEGO instructions such as: Rebrickable.com Worldbricks.com Brickshelf.com LEGO Digital Designer Gallery See Related Links!
Lego Halo does not even exist.
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There is no Lego Halo. If you have seen one, it would be 100% from a Lego fan. You will have to look around in Ebay, as Lego never made any Halo, or related sets.
Go to the link below and click on 'Lego' and then 'Halo Lego' and there it is the pelican dropship in Lego form + instructions
on ebay or online
walmart
You can get a normal lego figure paint it white then make halo armor and add a lego alien police helmet (blue one with adu on it) and then paint that white and paint a halo helmet
in a lego halo box or set
If you want instructions for any sort of Lego creation (that isn't owned by Lego) then I'd just search it and look at the first couple of links. I know that quite a few people have made Halo Lego on Moc-Pages and on personal sites. Try the related links below. Saber-Scorpion (need to buy instructions from store) Mocpages (doesn't have instructions) YouTube (IS instructions)
I don't know figure it out yourself