When the 接続助詞（せつぞくじょし） (conjunction particle) たら is used before the 副詞（ふくし） (adverb) どう, it primarily presents a situation, then asks a question about it potentially happening. This construction can be translated as ‘why don’t you (A)?’, or ‘what if you did (A)?’. Realistically, it just means ‘when (A), how about that?’. It is often used for giving advice.
To use this expression, convert the verb that you want to use into its たら form, and then add どう.

• 食（た）べる量（りょう）を減（へ）らしたらどう？
  Why don't you reduce the amount you eat?
• テレビがつかないの？叩（たた）いてみたらどう。
  Your TV doesn't turn on? Why don't you try hitting it?

Sometimes たら is used by itself, with a tone that implies that it is a question (or simply by adding a question mark in written language). This is an abbreviation of たらどう, and carries the same nuance.

• 暇（ひま）なら外（そと）に行（い）ったら？
  If you are bored, why don't you go outside?

だ or か may be added to the end of たらどう, to emphasize the ‘question’ even further. However, they are not required.

• 警察（けいさつ）に電話（でんわ）をしてみたらどうだ。
  Why don't you try calling the police?
• 苦（にが）いなら砂糖（さとう）を入（い）れたらどうか。
  If it is bitter, why don't you add some sugar?