Step 3: solve the second layer

Keep yellow on top. You're now placing the four edges of the middle layer, the ones with no yellow on them. When this step is done, the bottom two layers are solved.

How to do it

Find a top-layer edge without yellow.
Turn the top until the edge's front sticker matches the centre below it. You've made an upside-down "T" of one colour on the front.
Check the edge's top sticker.
- If it matches the right centre, the edge goes down to the right.
- If it matches the left centre, it goes down to the left.

Insert to the right

U R U' R' U' F' U F

Insert to the right: U R U' R' U' F' U F

Insert to the left

This is the mirror image of the right-hand insert.

U' L' U L U F U' F'

Insert to the left: U' L' U L U F U' F'

How to remember them. First, move the top away from the slot. Then bring the corner down and back up, which is the first-layer move you already know. Finally, do the same move again from the front to bring the edge in.

An edge is stuck in the middle layer

It's in the wrong slot, or in the right slot but flipped.

Hold the cube so the stuck edge is at the front-right.
Do the right-hand insert with any top edge. That pushes the stuck edge up into the top layer.
Place it normally.

U R U' R' U' F' U F U2 U R U' R' U' F' U F

Flipped edge at front-right: insert right to pop it out, then place it (U2, insert right)

When there's no edge without yellow on top

All remaining wrong edges are then stuck in the middle layer. Use the fix above.

Common mistakes

Matching the wrong sticker. Line up the edge's front sticker with the front centre, then read the top sticker to choose left or right.
Mixing up U and U'. The right-hand insert starts by turning the top away from the right, so U sends the front row to the left.
Breaking the first layer. If the white side gets messed up, one move was wrong. Undo it and redo the algorithm slowly.