Basically you want to create an oscillator (bandlimited) then feed that through a resonant filter, then to your speakers.
Bear in mind lop~ isn't resonant, other people have created externals that are, or there is the moog~, although this will sound nothing like the monotron filter, from what I remember Korgs might be 12dB/octave (might be wrong!). Good luck in designing your own Analogue modeling filter, I tried and it was solid!
To replicate the monotron ribbon controller, it's probably simplest to control the frequency of the oscillator
create another oscillator (a simple sine wave), this will be your LFO. connect that up to either filter cutoff or amplitude
Amplitude would require you to multiply the output of your main oscillator by the output of LFO
Cutoff would mean multiplying the LFO output by The value you are inputting into your filter cutoff input.
You could incorporate a button switch so you could flick between them like the monotron
It is a very simple synth to emulate, I recommend reading this site to explain the basics of subtractive synthesis in PD , it also mentions bandlimited oscillators, which will stop them aliasing THIS IS VERY IMPORTANT!
A guy from this forum also wrote this site
You will soon grow bored of replicating the monotron, and move into doing minimoog replications, and maybe even modular stuff.
I saw a video of someone who did a Swarmatron replication, very impressive. Wish I had that patch...