Not "by heart", "in" your heart.
I mean that both literally and figuratively.
If you are interested in doing a precise cover of the song, and if you haven't developed an ear for rhythm yet, then you will need someone to give you the rhythm. However, my opinion on precise covers is that there's not a lot of point to them except sometimes as a tribute. The original artist has already done the song that way. Besides, even original artists change the songs they perform and don't do straight album covers all the time. So the figurative part is that rhythm is in your heart. Play what you feel rather than trying to cover the original artist with precision. Play it the way you would have played it had you been the original artist. Some of my favorite songs that I play were originally written for fiddle, hammered dulcimer, banjo, mandolin, and piano. I don't make any attempt whatever to play them like I originally heard them on an album. I take the song and play it from my heart, within my skill level and in a way that makes it fun for me and hopefully for anyone unfortunate enough to hear me.
The literal part is that your heart is your guide to rhythm. If you are healthy, at rest you're going to have a pretty steady heart beat. Use that as a metronome of sorts. Get used to feeling your own pulse and how it relates to music. Get used to that PUMP pump PUMP pump PUMP pump PUMP pump. As you listen to music, see if you can hear it. ONE two THREE four ONE two THREE four (for common time songs) or ONE two three ONE two three ONE two three (for waltz time songs). As you begin to hear the heart beat of the music then it becomes a fairly simple matter of listening carefully for how many strummies are happening in each beat. I like Johnny Cash music, which has frequently been described as "the boom chicka boom sound." BOOM (he hits a bass note and that's the only note for the first PUMP or ONE of the measure), then chicka (he does a down strum, the chick part, and an up strum, the 'a' part). Both the 'chick' and the 'a' take place in the recovery 'pump' or 'two' beat. Then he picks an alternating bass note and does another down and up strum. He usually plays in 4/4 or common time, so each measure has four beats. The full description of the four beats is ONE two THREE four, or BASS NOTE down and up strum ALTERNATING BASS NOTE down and up strum. If you learn counting a measure it is ONE two-and THREE four-and ONE two-and THREE four-and. It takes exactly as long to say ONE as it does to say two-and. You have a steady beat. Just like your heart.
If using your heart is too difficult for you at first, and it is difficult, then purchase a metronome or use one on-line. If you can, get one that allows you to set how many beats per measure there are and gives an extra strong click on the first beat of the measure.
Soon, you will begin to pick up how many down strums (usually the stronger more full sounding strum) and up-strums (the higher pitched softer strum because many players only hit the treble strings on an upstrum) you hear for each click of the metronome or each beat of your heart. There's your rhythm.
Bear in mind that if you are listening to anything other than a solo artist playing only with an acoustic guitar, you are going to have to do as I suggest in the first part and modify what you hear. You cannot make a solo acoustic guitar sound like full instrumentation with drums, bass, saxophones, piano, harmonica, lead and rhythm guitars, backup singers, etc. A certain amount of bringing your heart to it (figuratively) is necessary. Then throw in that you're a beginner with 1/2 hour per day to practice and you are probably listening to exceptionally talented professional players, precise covers are unlikely. You would have to be a one in ten million natural player to be able to do that successfully. Don't set yourself up for frustration that way.
It just seemed quicker to suggest playing it from your heart as I did in my first message.
Congratulations on your child. I have two young ones at home myself. I enjoy playing and singing lullabies for them. The eldest is finally showing some interest in learning guitar herself.
- Zurf