So presumably you've unlocked Marco (allied with Sirius, got invitation and donated 25 modular terminals) and now you have access to his upgrades.
To go up engineering Grades (levels) you need to gain increasing reputation with that engineer. Check the Inara page for what each engineer will accept to increase rep (
https://inara.cz/engineer/7/). Marco will only go up to Grade 4, others can go to Grade 5.
Crafting modules to increase rep is the most obvious. You'll need to apply the engineers modification in iterations to unlock the next Grade. For example, choose Armoured Power Plant, apply Grade 1 in iterations until Grade 2 unlocks, then apply Grade 2 until Grade 3 unlocks, and so on. The next Grade usually unlocks (ie stops being greyed out) when you get the lower Grade to 80%.
When you are crafting modules to increase rep it will eat up lots of materials, however once you have unlocked an engineering Grade, the mats cost will be lower in future. Initially gaining rep at an engineer might cost you 5x the Grade 1 mats, however once you have fully rep'd up the cost will be more like 1x. Using the alternate rep gain mechanism can reduce your initial mat drain a lot, which for Marco is selling commodities.
I'm not sure if I've explained that well. It sounds complicated, but once you get the hang of the system its pretty straight forward.
Remember that you can "pin a blueprint" (only 1 per engineer) to apply the engineering at any station in the future. However experimental effects can only be done at the engineers base (or another engineer that does the same module).
A good resource:
https://www.edtutorials.com/engineering/engineering-explained-complete-process/