im far less qualified then those above me but ill do my best to try answer some questions:
What difficulty pk would you suggest someone makes as their first and why?
This ones unanimous, always go with an easier difficulty as they are far easier to work with: I will go one step further though and explain what you should aim for with each difficulty.
Easy: An easy map should be able to be beaten by anyone, even people who have just installed Minecraft and joined a server. They should be a map people can switch there brain off to and just blaze through and enjoy. Notably they should be easy to play without shifting or a sprint key to maximise accessibility. Aim for around 20-30 jumps with this.
Medium: A medium should be a slight step up from easy but not much, jumps that involve shifting and sprint keys are now on the table and it should do its best to pose a tiny threat, but still a threat to new players while being inviting to those who want a step up from the bliss of an easy. Aim for 30-50 jumps with this.
Hard: A hard is my personal opinion
the hardest difficulty of all of them to make, as such I would not recommend starting with this, these maps serve to bridge the gap from a pretty cosy ride that's a medium poses, to a decently intense ride that's an expert poses, getting that sweet spot while not being too easy or too overbearing is very difficult, and as such should be heavily experimented with to get something that is a fitting middle difficulty. Aim for 50-75 jumps with this.
Expert: An expert is the first difficulty locked behind a higher rank, so as such expect people to have played some of the above difficulties before this. These should essentially be a toned down insane, using similar gimmicks while being much nicer and safer with them, as such to sort of prepare people for the hardest difficulty. They should also build on hard maps while making it clear you aren't going to just blaze through this like before. Aim for 80-100 jumps with these.
Insane: An insane is the hardest difficulty and also by far the most wide spread of difficulty, maps here are no doubt challenging and daunting for players but there's much more experimentation and sort of unhinged design in them. Some insane maps will be a little step up from an expert, and some will kick your ass. Honestly for making it the best thing to do is make something challenging, but beatable. Something to challenge the best of players not through sheer jump spam like 8 quadruple neos in a row, but rather challenge through design and special gimmicks that make the map unique and stand as testing a different skill set than other maps. Aim for 100-140 if you're newer but push further if you think you can make the map not feel like it drags. 170-200 is where maps start feeling like marathons.
Any specific examples of maps to look at for examples of good pk (any diff)?
More recent maps typically reflect the server's standards much better, but also many lists like the ones pink has provided above help you find them, maps with higher ratings typically mean its better (not always so don't use ratings as gospel) but also just ask around as everyone has differing tastes in parkour and as such like and dislike different things.
What is your pk map making process?
I have very little building skill so I am unable to comment on that but I CAN comment on parkour.
To start you always path your map out, figure out where the jumps will go and the tour of the map you shall take, always make sure to patch exploits that may be in your path whether that be a build you can climb up or another exploit found later, make sure to use a decent chunk of the build as well.
Next start by selecting a palette, I personally like to go with blocks that match the color scheme of a map but you do what you like.
Next just, go for it. Look for jump inspiration online or on other servers or even on other peoples plots, though I personally like to make my own original jumps and don't like using others.
Next I'll build a jump/jumps and make sure the jump itself is fun to do. If you can't do your own map, there's something wrong there isn't there? I also put myself in the shoes of if I were to play this map, would I enjoy this? If the answer is no then change it till its yes!
I also make sure to take note of flow, does the map feel good to play in one go? How is it section to section? Keep these in mind.
After I build all the parkour and test it all to make sure it's fun, I'll put checkpoints in appropriate spots and make sure they're fairly placed and the maps good checkpoint to checkpoint.
What do you keep in mind the most when you make maps?
Design, players have different ways of thinking than you, account for it. Such as:
-Always have a failsafe in a room, like a ladder to get back out.
-Use visual indicators for where to go, don't use a sign that says "ladder behind here!" Put a glass block so they can actually SEE the ladder, people don't read signs so to maximise enjoyment always mark things visually instead of through written means.
Any other things to suggest?
You're not gonna get it right the first, second, or maybe even fifth time, but never give up cause one day you WILL get it.
Always listen to criticism, people will tell you things cause they wish to help you and make the map better. Listen and follow advice to maximise the potential of your map.
Make life for judges easier, they are just testers too after all. If you have a tp block, describe where or show where it leads to instead of just listing co-ordinates. They will change when your maps released.
Good luck and I hope you can be a great mapper one day