First, you must be familiar with inara.cz and coriolis.io sites. The best start would be to unlock Felicity Farseer and mod a ship with everything she has (FSD, thrusters, power plant, etc.). Read the description for each mod at inara.cz and make a list of materials you need.
Basically you will have to dedicate yourself to 2 activities: materials collection and engineers unlock/raise them to grade 5. There are a lot of ways to collect materials: missions that offer them as reward, killing other ships and collect the debris, collecting meteorites fragments with the SRV and special places (from volcanic to barnacles to brain trees or places like Dav's Hope, The Bug Killer, or Koli Discii). You also have to collect data by scanning ships or wakes or planetary/megaships installations data points (also crashed ships in the special places like Jameson's Cobra). You will also collect materials from USSes (Degrated Emisions, Encoded Emissions, High Grade Emissions, etc) that you encounter in supercruise (or by scanning the NAV beacon in a system or by using the FSS with the scale at the most left area).
After you have a lot of materials and engineers unlocked, you need to think what do you want to mod and what mod to choose, depending of your playing style. There are obvious mods (like increase FSD range + mass manager special effect or dirty drive tuning with drag drives), but there are also more specialized ones, like for weapons.
You may want to use a tool (like EDEngineer) that will show you in real time what you have and what you need for a certain blueprint.
You may treat engineers casual (and stick to Felicity for example) or unlock and mod everything to grade 5 on a ship. Just try to avoid the burnout if there are activities that you need to do while collection mats but you don't enjoy them. Don't do repetitive jobs more than a couple of hours and don't do them for many days in a row - you may feel you don't want to play the game anymore...
Some will say "just play the usual game you like and collect mats while doing this". I don't agree with that, engineers need a certain level of commitment if you want to fully modify to grade 5 a ship for a certain role. Unless you are ok to modify a ship per year.
The good news is the result is wonderful: I basically don't want to fly an non-engineered ship anymore. When I buy a new ship, I immediately apply the usual pinned blueprints to it and go to some engineers for special effects only. But this means that every once in a while I dedicate like a week-end to just collecting materials.