Here a few thoughts which might help you create a better guide,

-laning/jungle phase needs to be explained
-Thornmail isn't required when you get Frozen Heart/Randuin's Oomen (as said in earlier post)
-Try adding a Philo stone for more sustainability (or an early Growth Pendant for FoN)
-Make your yourself a target by getting Sheen/Triforce, since you have no taunt these will boost your dmg output a lot

I hope these little things will help you and will vote once I see what you did it

Your Marks sucks, please tell me how ~50 mana can solve your mana problems. Get magicpen to boost you dmg instead.

Thornmail is not a really good item to get with malphite, and even less with this build.
Reason: the damage return means you want them to attack you. You have no taunts, and you passively slow attack speed with Frozen Heart and actively with Ground Slam. Together, these minimize the damage portion of Thornmail, and you aren't making yourself a target in the first place.

You have very little offensive power with this build, and therefore no way to make the other team target you. You also have very little lane sustainability. With only a Doran's Shield for early regen, you will need to recall often due to either low hp, mana, or both.
For now, I am going to have to give you a -1

I think I would recommend you to not get Glacial Shard but get Cloak of Flames. Clearly, you have been playing him as a sub-tank/mild support character.