1. Explain it simply
不如 compares two things and says the first one loses: A 不如 B means A is not as good as B.
Add an adjective after B to name the quality being compared: A 不如 B + adjective, as in 坐公交车不如坐地铁快.
With no adjective, A 不如 B still means A is generally worse than B.
Keep the order A 不如 B; the thing that comes out better is always B, right after 不如.
2. Show examples
坐公交车不如坐地铁快。
Zuò gōngjiāochē bùrú zuò dìtiě kuài.
Taking the bus is not as fast as taking the subway.
不如 = the bus loses on speed
今天不如昨天热。
Jīntiān bùrú zuótiān rè.
Today is not as hot as yesterday.
the adjective 热 names the quality
我的手机不如你的新。
Wǒ de shǒujī bùrú nǐ de xīn.
My phone is not as new as yours.
B (你的) is the newer, better one
3. Share a tip
Put the better thing right after 不如 - say 坐公交车不如坐地铁快 when the subway is faster, not 坐地铁不如坐公交车快。