Like all of the greats, Lennon and of course a little band called The Beatles had an instantly recognisable sound. So if you're working away from your home studio and looking to lean on a bit of Lennon magic then here's 3 ways in which you can inject some of it into your mix.
Firsty, slapback delay which you'll hear on many, many of Lennon's vocal recordings. Most stock delay plugins will have a preset but you'll be looking around 40 - 100 milliseconds to get that signature sound.
Using an analogue style delay will get you even closer adding some saturated grit. If you want to go all in check out Studer tape machine plugins. Waves make a good one called J37 based on the famous Abbey Road tape machines and this will deliver Lennon-esque slapback and saturation in spades.
Next up, Lennon often double tracked his vocals to thicken them up. If you're not with the vocalist however you can take advantage of the digital age and simply copy the main vocal, drop it onto a new track and shift it along the time line by around 10ms. Play around with panning and use what works with the track.
Finally, The Beatles and Lennon were famously experimental when it came to studio effects. the Leslie Rotary speaker was one which he used on a number of tracks for his vocals to create that psychedelic, trippy, out-of-phase sound.