The art of Demonsteel is the 33rd quest in a 34 quest chain. If you didn't cancel or forget any quests you can simply open your questlog and complete the quests in the "Blacksmithing" tab. This will automatically continue the quest chain for you.
If you have skipped or cancelled a quest the only thing you can do is guess which one it was. Sometimes you'll see a yellow exclamation mark if you open the world map but it could also be that you have to remember where you left off.
Here you can find a Wowhead comment with all the quests in the chain. Theseare as follows:
ID Quest Zone
1 Strange New Ores Dalaran
2 The Properties of Leystone Dalaran
3 The Methods of the Nightfallen Azsuna
4 Hatecoil Hammerwork Azsuna
5 Engineers: Not COMPLETELY Useless Dalaran
6 Chicken Scratch Dalaran
7 Secrets of Leysmithing Dalaran
8 Nature Provides Dalaran
9 Leysmithing Mastery Dalaran
10 Flower-Pickers and Potion-Quaffers Dalaran
11 The Highmountain Smiths Highmountain
12 You Are Not Worthy Highmountain
13 Ironhorn Leysmithing Highmountain
14 Grayheft Highmountain
15 From One Master to Another Dalaran
16 Not Just Weapons and Armor Dalaran
17 Leystone Hoofplates Dalaran
18 Legend of Black Rook Hold Val'sharah
19 Between the Hammer... Val'sharah
20 ...And the Anvil Val'sharah
21 The Knowledge of Black Rook Dalaran
22 A Sweet Bargain Dalaran
23 Advanced Quenching Dalaran
24 Felsmith Nal'ryssa Suramar
25 Part of the Team Suramar
26 Smith Under Fire Suramar
27 Nal'ryssa's Technique Suramar
28 Leystone's Potential Suramar
29 The Firmament Stone Highmountain
30 Leystone Mastery Highmountain
31 Hammered By The Storm Maw of Souls
32 Worthy of the Stone Highmountain
33 The Art of Demonsteel Highmountain
34 Tribal Knowledge Suramar
You need level 110 or higher in blacksmithing in order to start Leystone mastery