Tunnels is a rather interesting part of the game. Navigating it isn't too tricky to figure out either as you are given a map in-game with your exact positioning and direction you're facing.

The goal here is to get 2 sets of flares and the compass so you can escape through the "non-blocked" exit.

The map provided courtesy of Dammit9X of Gamefaqs (all credits to him for the making of this) details all rooms you can visit in here as well as the exact location of each crucial item you need to advance the game to the next part.

https://www.gamefaqs.com/nes/587514-platoon/faqs/50974?raw=1

In a speedrun, your first step is to grab the first set of flares in the top left corner of the map provided above. This should go without saying since your starting location very close to it.

From there, it's debatable whether it's faster to go for the second set of flares first and then the compass or vice versa. Normally I go for the compass next but I've been tempted to try out the other way too so it's really up to you.