> The first thing you need is one line that goes high and low once
> a second

That is so easy. With hindsight !!

There are common ways to do it

(1) use a single crystal for clock and core
(2) separate crystals
(3) count mains cycles

An example of (1) would be to have, for example, a 19.6608MHz
on the OSC pins and use one of the PICs timers (or, more accurately,
hardware counters) to divide this frequency down

So, the PIC's Fosc will be 19,660,800/4 = 4,915,200Hz

If you use the 16-bit TMR1 as the divider, it's first loaded with 0x4000,
which equates to d49152 counts below the rollover of 0x0000. Therefore,
it takes 49152 transitions of Fosc for TMR1 to count up to 0x0000, and
TMR1IF to be set (which can be polled or used as an interrupt). As Fosc
is 4,915,200Hz, then this will happen 100 times/sec. That can be further
divided down with a RAM register to get 1Hz

Note that power consumption rises per MHz

TMR1 can also have its own crystal, independent of the core. Generally
this would be a 32768Hz "watch" crystal. Again, you pre-load TMR1
with a value to get the division you want. If you used TMR1 in 16-bit
free-run mode, it takes 65536 counts for a cycle, or 0.5Hz ( = 32768/
65536 = 1 rollover per 2s)

By loading TMR1 with 0x8000 then the output is 1Hz, because you're
starting the count up to 65536 at 32768

Counting mains cycles, if available, would be the best in terms of long-term
accuracy. The source AC used to power the PIC is fed to a PIC input, for
example an INT pin, and the transistions of it are counted in RAM. Divide
by 50 or 60 as appropriate to get the 1Hz

My own personal favourite, especially for battery-powered clocks and
loggers, is to use the module from a wall clock

http://home.clear.net.nz/pages/joecolquitt/coil-driver.gif

Wire up both outputs (they're reciprocating) for 1Hz. Good to a couple
of minutes / year

All except (1) can be used with reduced PIC power

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