How to use a metronome and count-in when recording
Press M in Loop by Zaweba for the metronome and set a count-in of 0 to 2 bars, so you start recording on the beat and stay in time.

Using a metronome and count-in when you record in Loop by Zaweba takes two settings. Press M to toggle the metronome, and set a count-in of 0, 1 or 2 bars before you press R to record. The count-in plays the click before recording starts so you know exactly where the first beat is, and the metronome keeps ticking while you play so you stay in time.
Both work for recording audio from a microphone or line input and for recording notes from a USB MIDI keyboard, the on-screen keyboard or your computer keys.
What each one does
The metronome is a click on every beat at the song's tempo. Loop's songs are in 4/4 with a constant tempo, so the click is steady from start to finish.
A count-in is a short run of clicks before recording begins. With a one-bar count-in you hear four clicks, then recording starts on the next beat. Two bars gives you eight clicks, which is useful when you need a moment to get your hands on the instrument after pressing R. Zero means recording starts straight away.
Setting up a recording
Add the track first. For your voice or a guitar, choose Track > Add audio track. For notes, use an instrument track such as the synth, sampler or a General MIDI instrument. Studio mode (F3) shows the recording settings among its tabs.
Press M so the metronome is on, choose your count-in, and move the playhead to just before the place you want to record. Press R. Listen to the count-in, then play.
If you want several attempts at the same section, press L to turn on the loop region around it. Every pass over the same place is kept as a take, and you choose between them later from the clip's right-click menu or the Instrument tab.
Hearing the click clearly
When you record a microphone, use headphones so the click doesn't leak into the recording. The metronome itself is never exported, but a click picked up by the mic would be part of your recorded audio.
If the click is hard to hear against the backing, lower the other tracks in the mixer while you record and bring them back afterwards.
When the timing still seems off
If everything you record lands a fraction late even though you played with the click, that is latency rather than your timing. Adjust the latency compensation setting and the buffer in Tools > Preferences, then do a quick test by clapping along to the metronome and looking where the claps land on the grid.
For MIDI recordings, quantise in the piano roll tightens notes to the grid afterwards. Choose a grid value that matches the part, from 1/4 down to 1/32, triplets included.
Do you always need a click?
Not always. If the song already has a steady drum part, many people find it easier to play along with the drums and switch the metronome off. The count-in is still handy then, so you don't miss the first beat.
Playing without any click at all suits loose, free sections, but remember that the grid, the loops and the exports all assume the song's tempo. Parts recorded off the click won't line up with bars, which makes copying and repeating them harder.
Good habits
Practise along to the click a couple of times before pressing R. Keep the count-in the same for every take of a part so the start always feels the same. And save between takes with Ctrl+S, so a good performance is safe as soon as you have it.