A cone in 5e is defined such that

A cone's width at a given point along its length is equal to that point's distance from the point of origin.

That is a 53-degree cone, not a 90-degree cone as 3.5e used. Thus the diagrams given from the Pathfinder SRD are not applicable.

If you don't want to just eyeball it you have 2 choices:

1. (requires advance preparation) Create a scale template on a piece of 1" grid paper. Overlay it on your battle grid in the direction your caster wants to direct their grid. If more than half a square is covered, consider the square to be affected
2. (easier to do on the fly) Create two "measuring sticks" (strip of paper or whatever) scaled to the map grid, to represent 15' lengths. Place one extending from the edge or corner of the caster's square in the direction the caster wishes to aim the spell. Place the other at the far end at 90 degrees which represents the maximum width of the cone at its end. Draw the imaginary diagonals from the ends of that width measure back to the origin.

Both of these methods allow the caster more flexibility than the pre-gridded approaches. The caster can aim the spell in any direction they like (rather than just the 8 compass points and diagonals) to attempt to hit as many enemies and avoid as many friends as possible.

Once you've done it a few times, you'll probably be able to eyeball it in all but the most complex battles.

A cone in 5e is defined such that

A cone's width at a given point along its length is equal to that point's distance from the point of origin.

That is a 53-degree cone, not a 90-degree cone as 3.5e used. Thus the diagrams given from the Pathfinder SRD are not applicable.

If you don't want to just eyeball it you have 2 choices:

1. (requires advance preparation) Create a scale template on a piece of 1" grid paper. Overlay it on your battle grid in the direction your caster wants to direct their cone. If more than half a square is covered, consider the square to be affected
2. (easier to do on the fly) Create two "measuring sticks" (strip of paper or whatever) scaled to the map grid, to represent 15' lengths. Place one extending from the edge or corner of the caster's square in the direction the caster wishes to aim the spell. Place the other at the far end at 90 degrees which represents the maximum width of the cone at its end. Draw the imaginary diagonals from the ends of that width measure back to the origin.

Both of these methods allow the caster more flexibility than the pre-gridded approaches. The caster can aim the spell in any direction they like (rather than just the 8 compass points and diagonals) to attempt to hit as many enemies and avoid as many friends as possible.

Once you've done it a few times, you'll probably be able to eyeball it in all but the most complex battles.