How can I reliably place C4 on my enemies?

I've got bad news for you: you have to be really close to someone to be able to place C4 on them. How close, you ask? It looks like it actually depends on the opponent's hurtbox--meaning the smaller the character, the closer you'll have to be.

On the training stage with the opponent centered on 0, the furthest I could be was 7 "units"¹ for Pichu (one of the smallest in the game):

but 12.5 "units" from Ridley (one of the largest):

These distances also differ when Pichu/Ridley were facing the other way, making me think that this is indeed based on hurtbox size.

So your problem is likely that you're simply not close enough.

You could try to get in and find an opening where your opponent gives you enough time (9 frames) to plant the C4 on them, but then you're almost better off doing something like forward-tilt/up-tilt/grab which all come out faster or even forward-smash which is only 2 frames slower.

Instead, you can use the fact that C4 will still be planted to the opponent even if they're shielding!

So I'd say the most opportune time to use this would be after you've conditioned them to shield when you approach. This way, you can use whatever approach you've been, but mix up the attack for a run up sticky.

Minor trivia I learned while experimenting

• Dropping a C4 from above will always make it stick to the floor. Even if you drop it directly over the opponent (much to my dismay--this was going to be my suggestion if it worked).

• If a character is "stuck" with the C4 and they pass another character, the C4 will transfer to the new character, even if this character is Snake himself! So be careful after you stick someone or when you detonate, you may be killing yourself! However, after a short window (I estimated about 20 frames), the C4 can be passed again. So even if it gets passed back onto you, you could try to stick it to the opponent again by getting close to them.

• The C4 will fall off of the character after a certain amount of time, so if it gets transferred back to you, don't fret. You'll just need to wait for it to fall off.

• Shielding can change the size of the opponent's hurtbox, so the "stick range" may change slightly if they're shielding--either way, I'd suggest playing safe and getting as close as you can.

• Pichu puts her ears back when shielding to fit into the shield and it's the most adorable thing.

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^{1. I'm calling each of the smallest squares a "unit" here.}

Facts about the move:

• The C4 is 'stuck' on frame 9 to a fighter.
• sticking a C4 goes through shields, this cannot be parried (like a command grab, such Bowser's Flying Slam)
• range is very small as pointed out in the previous answer scohe001
• when the red light stops blinking, the c4 can be transferred to another. During the time the light blinks (about 2 seconds), it cannot be transferred
• the C4 explodes by itself after 1600 frames (roughly after 27 seconds)

With this knowledge in mind:

• If they shield a lot, run up and stick them.
• Stages with platforms like Battlefield; an opponent above you shielding can still get a C4 stuck.
• As senpai pointed out, Snakes down throw forces a situation where the opponent has to getup, roll, or getup-attack. This is the only throw in the game that forces this (cannot be teched) and can set up a situation where you can stick the opponent if you read correctly.
• Also worth noting is that damage scales the amount of frame advantage Snake has with down throw. See this video for more information on that. You can use those extra frames of advantage to read where your opponent goes and stick C4 on them.

As a side note, if you've stuck your opponent with C4 or if you are about to transfer C4 on them, a 100% true KO confirm is Back-throw -> C4 detonation.

I have noticed from personal experience (not confirmed) that a stuck C4 can fall off after being hit with a strong move, and it tends to happen more when the damage % on the stuck fighter is lower.

The easiest way to stick C4 on an enemy is after performing a down grab -- aka, Snake's sleeper hold. This works more reliably at high percentages as the down throw "stunlock" animation plays out a little longer and opponents have to mash harder to get out.