The Invincible

There are 6 memories in total with some of them missable

Memory 1
At the very beginning of the game at the start point head to the nearby water reservoir which should be on your left side as you head towards The Needle. Click the icon in the water and and after that a second icon will appear which will trigger the first memory.

Memory 2
Continue the story through and past the sandstorm, at this point you will reach rocks that you need to climb and at the very top you will look into the sky and see R3-03, simply click it to activate memory number two.

Memory 3
Immediately after the second memory you will make your way to the camp which is just around the corner and gain you The Camp achievement. As you enter the camp there will be a tent on the very left side nearest you, inside will be a cylinder containing a fish; click on it to activate the third memory.

Memory 4
After you're done with The Camp area move on to the excavation site and in the central section below the metal structure will be a mining drill making a loud noise, simply click it to activate the fourth memory.

Memory 5
After fixing the broken probe you will move into a lava area where you need to highlight each point 1-5 on your map. When you finally reach point 5 at the black trees continue ahead and into the tunnel, this will trigger memory 5 which this time you will need to navigate through as part of the story and can't be missed.

Memory 6
After the alliance field base you will follow the convoy path on your map, upon doing so you will eventually reach the robots. Continue all the way through this part until you come to a large shielded area where the story takes you.
After dropping the shield you should then be able to enter the TMU-88 vehicle and follow the dialogue. Whatever you choose doesn't matter here as all options will lead you into the final memory number 6 and complete this achievement.

If you need more help with this you can follow my video guide below:

There are 3 missable memories you must find and 3 automatic ones.
Here's my video guide for this:

Missable

To get this one you need to trigger all six flashbacks, three of them are missable
It has to be done in one playthrough but you can fast forward them.

First memory is triggered by interacting with the small lake at the very beginning of the game, just after picking up your beacon

Second one is story related, you'll get it when clibing up to the camp

Third memory is a missable one, you have to check on the fish in the camp. It is in a tank in a small tent.

Fourth memory is the last missable , just after exiting the camp, you'll find a metallic stucture. At its base, there is a portable drill that you do have to inspect.

Fifth and sixth memory are story related, one is triggered when walking throw the bushes when looking for Gorsky at the 'city' entrance. The other one will trigger just after encountering the flies at the convoy

Hope it helps !

Here is a video guide which i do not take credit