I assume you're using Windows? If so, first off, download and install
foobar2000.
Next, download the
LAME MP3 encoder and put it in your encoder directory (something like C:\Program Files (x86)\foobar2000\encoder or wherever you installed the programme to).
Now open the cue sheet with foobar, choose the track(s) you want to convert (right click -> convert), and choose "..."
Click output format and you'll be presented with a window shown in the picture below, the one on the left. If no option is present yet, add a new one for MP3 as shown in the right window.
I recommend setting it to V0 VBR (variable bit rate). Avoid 320kbps CBR (constant bitrate); just because it's the choice placed all the way to the right doesn't mean it's the best one. 320kbps CBR encodes consume more space for little gain (to the point of no discernible difference, especially not if you don't have the right equipment―and by that I mean thousands of dollars' worth of audio hardware). This is also how we do this at ASL (Anime-Sharing Lossless, our music group). For a more elaborate (technical) definition, take a look at our FAQ item outlined at the ASL blog:
http://koe.anime-sharing.com/faq/#faq-item-63
In any case, once you've made a preset for V0 VBR MP3, you're good to go. Other stuff such as Destination and Processing have no bearing on the transcoding/converting process itself and should be quite self-explanatory. For output formatting, you can refer to
http://wiki.hydrogenaudio.org/index.php?title=Foobar2000:Title_Formatting_Reference
Now, get back to the track you wanted to convert and get converting. Your new preset should appear under the right-click -> convert menu. You may be prompted to locate the MP3 encoder before the process starts. Point foobar to where you placed lame.exe and voila.
By the way, thread moved to tech section as this is more about questions about a programme than a request
for one.