what are the earliest known large scale maps (on a continental level), how did people first go about mapping large areas?

The earliest continent-scale map I know of is the atlas compiled by Eratosthenes in the 3rd century BCE. It doesn't survive, but we have a fair amount of information about it: you can find out everything that's known in Duane W. Roller's edition of the fragments, Eratosthenes' Geography (2010).

In Eratosthenes' time it was a more haphazard affair than in modernity, as you might expect. The main obstacle was the standard of surveying, and the resulting problems in quantifying the distances between places. Units of distance were not fully standardised -- we still don't know for sure which stadion Eratosthenes used as his basic unit of measurement: the Olympic stadion and the Ptolemaic stadion were different (ca. 185 m and 210 m respectively), and one source tells us that Eratosthenes used a stadion that wasn't equivalent to either of them (Pliny Natural History 12.53: in his account the Eratosthenean stadion comes out as 157.5 m). Another source (Strabo 17.1.24) tells us that a schoinos, nominally 40 stadia, could in reality vary between 30 and 120 stadia. Later on the Roman mile came along and helped standardise things a great deal, but Eratosthenes couldn't take advantage of that. We do know that the Ptolemaic surveys that Eratosthenes had access to gave the distance from Alexandria to Syene, and Syene to Meroë, both as 5000 stadia, but we don't know whether that's meant to be as the crow flies or along the route of the Nile; an added complication is that Eratosthenes treated them as being on the same meridian, when they're not quite (Syene is actually about 150 km east of the Alexandria meridian).

And that brings us to the biggest problem: longitude. Basically, the only way for Eratosthenes to calculate longitude was by using the rather inaccurate surveying data of the 3rd century BCE.

Latitude, however, was a doddle. No later than the early 3rd century BCE, government surveyors like Philon (a Ptolemaic official) had started taking measurements of the ratio of a gnomon to its shadow at midday on the solstices and equinoxes, as a way of ascertaining latitude. The resulting measurements had reasonable precision, and served as the basis for Eratosthenes' work. This is what allowed him, for example, to calculate the circumference of the earth on the assumption that Syene was equidistant between Alexandria and Meroë (the actual latitudinal differences between Alexandria-Syene and Syene-Meroë differ by about 5%).

We do have the atlas of Ptolemy, 400 years later, if only in textual form. Unfortunately, attempts to realise Ptolemy's map in the early modern period were hampered by extremely inadequate texts, and by Ptolemy's miscalculation of the earth's size and his choice of meridian as outlined in this very interesting 2013 study by Tupikova and Geus. Still, here's an interactive map based on Ptolemy, based on the 2002 Berggren & Jones translation of the Geographia; of course that translation doesn't take account of the new Stückelberger & Grasshoff edition, or of the adjustments that need to be made based on Tupikova & Geus. Still, it's good for exploring. The implication of Tupikova and Geus's study is that Ptolemy, at least, had access to rather good surveying data.