Tuners are nice, and there are some good cheap ones out there. They're good if you only play home, but if you take your guitar anywhere else to play (lessons, friend's house, camping, church etc) you will forget the tuner now and again--then what?
If you learn the 55545 string-to-string method, you only need one pitch to tune, then you can get the whole guitar tuned. I used to carry a single-note E pitch pipe, use it to tune the E string, then tuned the guitar with 55545 string to string. A single pitch pipe is about 1/4" diameter x 1" or so long, so I kept it tucked in the overloop of my guitar strap. Case pocket works well too.
Even better...I've been playing acoustic off and on for about 40 years, and I always recommend learning relative string tuning using harmonics. which are much higher pitch tones, but it's much easier to hear when the two strings' harmonics are in phase with each other. That's because you're only hearing one harmonic tone on each string, as opposed to the entire string note with multiple harmonics. Takes a little practice, but it works better. Also makes you look like you really know what you're doing.
Here's a quick find on YT that explains it well. Good stuff starts about 1:00 in.
I'm assuming you're playing a decent guitar that is well adjusted for pitch up the neck, at least to 12th fret. On a cheap guitar, or one that's out of adjustment, the harmonic tuning may not sound in tune on the lower frets. In that case, go back to 55545 or the tuner.
Now, if you really want an interesting but somewhat deep dive on guitar tuning, here's James Taylor on how he tunes his guitar. Interesting stuff.
