To say two things are the same in some way, put the describing word between two as: as + word + as. The word stays plain, with no -er.
as + word + as
Amina is as tall as Lars.
This book is as interesting as that one.
Hiroshi runs as fast as his brother.
My bag is as heavy as yours.
Not "Amina is as taller as Lars." — say "Amina is as tall as Lars.". Inside as ... as, the word stays plain: as tall as, not "as taller as".
Try it: Hiroshi runs ___ his brother. (as fast as / as faster as / as fast than) Answer: as fast as — The word between stays plain: as fast as (not "as faster as"). And both sides are as, so the second one is as, not "than": as fast as his brother.
not as ... as
To say one thing is less than another, add not: not as + word + as.
She is not as tall as her sister. (= shorter)
Today isn't as cold as yesterday. (= warmer)
This cafe isn't as busy as the one downtown.
Try it: Today isn't ___ yesterday. It's warmer. (as cold as / as colder as / as cold than) Answer: as cold as — Keep the same as ... as, just add the not ("isn't"): isn't as cold as yesterday. The word stays plain (not "colder"), and both sides are as, so the second one is as too, not "than".
as ... as, or -er than?
Before you choose, ask are they the same, or is one more? The same → as + plain word + as. One is more → -er / more ... than. One is less → not as ... as.
The same: Amina is as tall as Lars. (they measure the same)
One is more: Amina is taller than Lars.
One is less: Amina is not as tall as Lars. (= she is shorter)
Watch the word: it stays plain inside as ... as (as tall as), but takes -er with than (taller than).
Not "Amina is as tall than Lars." — say "Amina is as tall as Lars. / Amina is taller than Lars.". as goes with as, and -er goes with than. Do not mix the two.
Try it: Mei and Sofia are both exactly 1.65 meters tall. Neither one is taller. Which one fits two things that measure the same? (Mei is as tall as Sofia. / Mei is taller than Sofia.) Answer: Mei is as tall as Sofia. — They measure the same, so use as + the plain word + as: as tall as. "Taller than" would mean one of them is more, but neither is.
Try it: My phone is ___ yours, but it isn't ___ that new one. (as new as … as expensive as / as newer as … as expensive than / as new than … as expensive as) Answer: as new as … as expensive as — Both blanks use the same pattern: as + plain word + as. So as new as and as expensive as. Keep the word plain and both sides as, in the positive and after "isn't" alike.