Since the acapella isusually the most important component of the recording of any song, it has to be exactly right. But getting it exactly right is the most difficult part of the entire recording process. Singers come in three basic types: First there is the top class performer who always gets it right, and the only other possibility is that he or she might go for another take and sing even better. Among people who call themselves singers, this type is one in a thousand! Usually you will be working with the second type who is someone who obviously can sing, but under the studio spotlight many defects become significant. The third category of singer is someone who has been chosen for his or her looks or personality and sings like a donkey. Well... You're in big trouble here, but you've got to pull through!
There are certain actions you can take to make sure the acapella is recorded as well as possible. Let's start with things that help even the best singers, and then work up to the more drastic solutions.
It isn't widely appreciated, but even the top singers need help to allow them to sing the way they do. We take it for granted that a champion tennis player needs a coach. So a top singer needs a coach too. There are two types of people who work with singers to improve their performance. One is a singing teacher who will help with the production of the sound, and the other is a voice coach who will help with the performance of a song. For our requirements, a basic singing teacher is probably the best option. (Bear in mind that some people who advertise themselves as 'voice coaches' work with speech rather than singing, so be careful not to get confused).
With the help of the engineer you will select a microphone that works well with the singer's voice, and sort out other technical matters. Many producers these days record a number of takes of the acapella, maybe as many as six or eight, and then sort through them later to get the best version of each line. With a little bit of skill you and the engineer will be able to compile the best of all of these versions, perhaps to the extent of swapping between takes on every line, or even the odd word where necessary. After this, there may still be the odd line that doesn't sound quite right, so you'll have to get the vocalist back in again to correct it. You may worry that since the time required to compile several takes into one can be quite lengthy, the vocalist may come back with a different tone of voice. Indeed, this may be a problem, but it is better than leaving a line in an unusable state. If you listen very carefully to records made by people chosen by the record company for their looks or personality, you will almost certainly be able to hear these inconsistencies. If it were possible to record a perfect acapella in one take, everyone would do it that way. Unfortunately the singers who are capable of doing this are, as said previously, very few and far between.
First and foremost, you need a multitracking application. This is the program you will record each individual instrument into to further process and mix your music. These applications come in all prices ranges, starting at free and going up to hundred, sometimes thousands of dollars if you are wanting a full hardware/software solution from somebody like DigiDesign, makers of ProTools or Steinberg, makers of Nuendo Systems. Quite often, however, some sort of a "light version" of one of these apps will come bundled with your sound device, which you can use to at least get a feel of recording on your PC. This is a good way to learn what you like and dislike about how the software works, and when it comes time to purchase an app, you will have a better idea of what you want.
If the worst comes to the worst and you just can't get a satisfactory recording, then you will have to use a little studio trickery - or a lot of it if necessary. This can range from double tracking, pitch shifting out of tune notes using a Harmonizer, or sampling a line and tweaking the pitch bend control as you record it back to tape, to a total reconstruction of the acapella using an audio sequencer with time and pitch correction software. If you think this is extreme, then be assured you that it is in fact normal to do anything that is necessary to achieve a perfect acapella, because that is what is going to sell the song. If you don't have the technical skills yourself to take on these tasks, sit back and let the engineer or programmer do it for you. All you have to do is decide when it's right!