"Have at least 600 scrap in your ship storage."
Accumulate a total of 600 scrap or more at one time. Money earned and spent does not accumulate towards the total.
As are all in-game achievements, this is most expediently achieved on 'easy' difficulty, mostly on account of increased scrap rewards. Every Scrap Recovery Arm augmentation you equip stacks another 15% increase on rewards from all sources as well. Instead of rushing, it is advisable to explore every sector as thoroughly as possible, to maximumize your profits from the multiplicity of beacons. Boarding ships rather than destroying them also tends to reap greater rewards. As you progress, each sector yields higher rewards, so make sure to capitalize on that.
Where practical, saving your scrap is just as important as earning more of it. Once you've reached a manageable shields and weapons level, good enough to come out victorious and unscathed in most battles, stop investing until it is absolutely necessary again. In stores, buy only what you need: Crew, weapons, and fuel should be considered your lowest priorities, as all of those can, at some point, be harvested from random events. If you chance upon any Zoltan crewmembers, they can even supplement a few reactor upgrades.
After acquiring this achievement, go on a shopping spree.