In western United States English, t's in the middle of a word are pronounced like a d. Ledder.
When you come across a tt in a word you're safe enough to pronounce it as one t.
The first t isn't fully pronounced anyway, the sound comes more from the second t.
As a kid I used to seperate tt letters...in this case I'd do Let + ter, and pronounce them seperately until they combined.
The 't' in let wouldn't be fully pronounced, it's like I'm stopping myself from letting the full sound of the 't' from coming out my mouth, whereas in ter, the 't' is more obvious.
It's actually pronounced as if there were only one 't'. The 'tt' is more for spelling, not pronounciation.
You pronouce it as if it was an 'r'. So letter would be pronouced something like 'lerer'