Wondering if anyone could help me with placing markers on my levels.

I first saw this in Resist Revolution.
https://www.youtube.com/watch?v=Mx1SXm7Z6_8&t=185s

I was hoping someone could point me in the right direction on how to make the markers in this game. eg: 1, 2, 3

In the markers tab, I believe your looking for the player checkpoint. It is the oragne one.
It is a marker zone with the checkpoint.lua script assigned to it.

Set them in your level along your playing route. Note that the player must run thru the zone. It is adjustable by moving the star points.

That,s not it, sorry for my wording.
from the video you can see the points the player needs to go to complete the task given.
It gives the player points to goto [1], [2], [3]

objective markers will need to be scripted, there's a few around but really this is quite hard to do in a general sense as it will require at least some input from the end user.
a basic script on a decal/object would work pretty well though (an invisible object can be used in place of an actual model), just something to check the player distance to remove it, you can spawn them in sequence using the ActivateIfUsed(e) fields.
i..e something like this

though this uses the script cut-off range so doesn't allow for distant markers very well (which is why marker [3] doesn't appear til i get closer) - probably better to use some hard coded toggles and allow alwaysactive = yes... though as i said above, this is far easier for you to set up than someone else.

for mine you set the 'name' to the marker prompt to show on screen and the 'health' / 'strength' to the distance the player needs to be to trigger the next one.