How do I produce steel in Kittens?

Steel is an item crafted in the Workshop, requiring 100 Iron and 100 Coal for each craft. Like all other Workshop crafting items, it will appear in the crafting menu on the left side once you have any amount of it.

Once you research either the 'Deep Mining' or 'Coal Furnace' upgrades from the workshop tab your game will start generating coal ('Deep Mining' produces it from mineral mining and 'Coal Furnace' produces it while you smelt iron).

Once you have enough (100 of each) coal and iron in your inventory, switch to the 'Workshop' tab, scroll down to the 'crafting' box and then click the 'steel' button to make your first steel.

Once you have some steel from that, you can either return to the same location to make more steel, or else click on the number links that appears to the right of the 'steel:' inventory item located in the lower left corner of your main screen. See the example screen below:

Click on one of the two circled locations above to make steel. Note the Workshop Tab.