Once you have posted, how do you pick a freelancer? Well, there are a lot of freelancers out there and there are also a lot of salesmen and then there are a lot of people who need money and don't have a clue about either. You need to learn to tell the difference. I can't tell you how many times I have looked on a freelancer profile and they are bragging about how they did this or that. When you check, you can't find their name (and sometimes you can't find the project they say they worked on) anywhere. You don't know if it was done as ghostwritten work or not. Their bid may be from a template (which doesn't necessarily mean they are bad) and their work posted may be plagiarized. How can you tell which freelancer to hire?
The first thing you should do is look at their posted profile (not how much they said they would charge). This is their resume. On every profile, there will be feedback (unless they are new). If there is no feedback, they could still be a good freelancer, but this means you are taking more of a chance. A freelancer with no feedback should expect to get paid much less than a freelancer with years of feedback. If the freelancer only has a couple of feedback posts, they are newer at freelancing (at least on that website) and should have 4-5 stars in everything. Freelancers with a longer history may have a few lower reviews. If all the reviews (or most of them) say the same thing, chances are good that is how the freelancer is. Can you live with a freelancer like that?
The more work samples they have to post, the more things they have done. Do you like their samples? When you look at their profile, do you think they have a good representation of the work you are looking for? For example, say you need someone to design a webpage for you and you go on their profile and see only samples and information about designing clothes with nothing about website design. That is probably not the freelancer for you.
If there profile is not set up, even with good feedback, that is a warning sign. People who freelance for a living take the time to add samples and make their profiles look nice. Again, you can find a good freelancer with no work samples, a poorly set up profile, and no feedback, but you are taking a risk. If your goal is to hire a good freelancer, you should toss any that don't meet this criteria. Now, if a freelancer claims to be working for years on another website, this person should have a ton of work samples to show you. They should be able to send you a message with a link to the other website profile on it to back this up.
Some freelancer websites also offer proficiency tests and the freelancers can post their scores on their profile. How do their scores compare to others on the same website?
Once you have screened out any candidates with the above, put their name into a search engine. Most freelance work is done under NDAs or ghostwritten, but established freelancers should have other work out there. Do they have a book on Amazon? Check to see if it was traditionally published or self-published. There is nothing wrong with self-publishing (or even vanity presses)- as long as the freelancer isn't trying to pass it off as traditional publishing (i.e. that freelancer started his or her own publishing company and then used CreateSpace or Kindle or Nook and published it). If you haven't heard of the publisher, you can do a web search on them(unless the freelancer told you they were self-published). Be wary of a freelancer publishing under another name with no way to link that name to his or her real name. Also, be wary of freelancers trying to get you to go against the website's terms of service (for example, if they bid under $25 on Guru or if they try to get you to pay them offsite).
After all this, you should have narrowed the applicants down. The next step is to ask the remaining freelancers a question about the project. How they answer the question should let you know how familiar they are with the project premise and also how well they communicate. Can you deal with their communication style? If not, toss them from the pile. Be careful that they are really answering your question and not responding with fluff and salespeak. Not communicating in clear English should not necessarily toss the candidate out (unless you are trying to hire someone to write in clear English for you). However, you should be able to understand the candidate.
Once you have gone through all this, now you can look at their bid. Unless you were extremely vague in your project description, the freelancer should tell you about how long they think it would take them to do a project as described and about how much they would charge to do this project. They should also outline what they will do that sets them above the rest. Did they send a sample of a project similar to yours- this should be looked at as a bonus (although because of NDAs and ghostwriting a freelancer may not have a sample of the right kind of work or even be able to tell you where they worked on something similar). Now you can compare the freelancers you prefer.
Why go through all this to find a good freelancer? Because once you have found one, you can return to him/her whenever you need that kind of work (and potentially other work if they have it listed on his/her profile). You can build a team. Most freelancers prefer working with regular employers and working with a freelancer repeatedly means you no longer have to go through this process because you already know you work well together.