Do Not put it in the sun
It will warp unless you stand there and flip it every 20 minutes.
Put it up off the ground on blocks, in a dry working environment on leveled blocks and then put weights on it to keep it level, straight, or flat. Level or straight or flat ....is very important. Depending on the thickness, it may take 2 weeks per inch of thickness to "acclimate" to your working environment, in my experience.
If it's an old board/plank it won't take long. If it's just been milled off the log, then it may take up to 1 year to dry and acclimate.
The answer to your question will only be as detailed and specific as the question is detailed and specific. Good questions also include a sketch or a photo that illustrates your issue. (:< D)