1. Explain it simply
A 没有 B 那么/这么 + adjective means A is not as (adjective) as B.
Use 没有, not 不, to make this comparison: 今天没有昨天那么冷.
那么/这么 sits right before the adjective and marks B's degree.
This is the negative side of a 比 comparison: 没有 says A falls short of B.
2. Show examples
今天没有昨天那么冷。
Jīntiān méiyǒu zuótiān nàme lěng.
Today is not as cold as yesterday.
没有 makes the negative comparison
我没有他那么高。
Wǒ méiyǒu tā nàme gāo.
I am not as tall as him.
那么 comes right before 高
坐公交车没有坐地铁这么快。
Zuò gōngjiāochē méiyǒu zuò dìtiě zhème kuài.
Taking the bus is not as fast as taking the subway.
这么 works like 那么 here
3. Share a tip
Use 没有 for this comparison, not 不 - say 今天没有昨天那么冷, not 今天不昨天冷。