Each syllable is a square
Hangul gathers the initial, the vowel and the final consonant into one square block. A syllable with a final consonant and one without should fit the same block height, so the line looks even.
Where the vowel sits
Vertical vowels (ㅏ ㅓ ㅣ) sit to the right of the consonant; horizontal vowels (ㅗ ㅜ ㅡ) sit below it. When a final consonant is added, the consonant and vowel move up and get a little smaller.
Stroke order basics
Write from top to bottom and from left to right. ㄱ is one stroke, across and then bent downward; ㅇ is one round stroke; ㅅ is the left stroke and then the right; ㅎ is the short top stroke, the horizontal, and then the circle.
Keeping the line
Imagine an invisible baseline, and keep the height of the characters and the space between them constant. Lined notebooks and manuscript grids help.
How to practise
Write one syllable three times and compare the first with the third. Writing slowly, one syllable at a time, tends to help more than writing a lot, fast.
About the practice squares on this site
The practice squares on this site are for Japanese kana and kanji; Hangul strokes are not judged yet. To try Japanese characters, use the square in the kanji stroke order guide.