Start with the goal in one sentence, then add the numbers. Write your current rank and division, the target you have in mind, the number of matches or sessions you expect and any date the goal has to be reached by. Boosters price what they can measure, so the clearer the numbers, the more accurate and comparable the offers will be.
Next, decide how you want the work done. Solo means the booster plays on your account while you rest, which is the fastest option. Duo means you play every match yourself with the booster in your party, so the account never leaves your hands, though higher ranks limit how far apart the ranks in a party may be. Coaching means you do the playing and the booster teaches. If you are unsure, say so and ask boosters to recommend one.
Then list the details a booster needs to know: your platform, your server region, the heroes you want or do not want played, hours when you need the account yourself and anything else that matters to you. Mention whether your account has reached level 15 and unlocked competitive mode, because some requests depend on it. Details like these prevent misunderstandings after the order has started.
After posting, read the offers carefully, including the short messages boosters add to them, and do not hesitate to ask questions before you accept. Once you pick an offer, keep an eye on the chat at the start of the order, especially on the first login, which may need a verification code. From then on the booster keeps you posted in the chat, and when the goal is reached, you confirm the result and the order is complete.