This is a collection of tips that have been discovered by Scott, RJH and Lucky since they've been making RPL files for the community. RPL files provide collections of presets in Reaper's native format, which is what gets displayed in the Presets combo box of the FX window, and is often more accessible than the custom preset browsers currently found in plug-ins and software instruments.
The idea behind providing an RPL file is that one person can do the initial grunt-work of hacking their way through the inaccessible preset browser of the plug-in or instrument, re-saving the factory presets in the more accessible native RPL format so that everyone in the community can benefit from those presets.
Typical tricks for that initial scavenge are to use OCR, sometimes trigger presets via MIDI program changes, or procure a spot of sighted assistance to click through the inaccessible bits of the plug-in or instrument GUI.

Follow these 3 steps to adopt what has become our in-house style for community resources:

1. The first thing we do when starting a new RPL is save a preset with the plug-in or instrument in its default state (just however it loads up).
We call this first preset "0 Default Settings", starting the preset name with 0 so that users can always return to default settings quickly by hitting 0 when in the Presets combo box.

2. Now use whichever method you've found to start stepping through the presets of the plug-in in chronological order, resaving each one using REAPER's "Save preset" option. That option is found in the context menu when you hit the "Preset and Performance Management" button in Reaper's FX window.
For sighted folk helping out, that button is labeled with a Plus "+" symbol visually.

Note that RPL format doesn't support folders, so here are some tips on how we tend to create the ilusion of those in the RPL versions.
Often, plug-ins will provide an easy way to reference the folder structure being presented in its inaccessible preset browser somewhere in a supporting directory, such as Documents\Vendor\Plug-in\Presets to take a commonly used path. If such a reference is available, then we add dummy place-holder presets in situ as we're resaving. To do that, we hit 0 in the Presets combo box to reset the plug-in to its default state, then re-save those settings with a new preset name that reflects the folder, prepending a number or symbol to provide easy first letter navigation.
Example fake navigation for a reverb plug-in  might look something like:
"1 Rooms (FOLDER)" for a main category
"- Small Rooms (SUBFOLDER)" for a sub category.
"- Huge Rooms (SUBFOLDER)" another sub category.
"2 Plates (FOLDER)" the start of the next main category
Typically, we reset to default state before saving each of these dummy navigation presets above, so that users can jump through our fake replica folder structure quickly without the plug-in or instrument updating settings on the fly as they move through available categories. This step is only necessary because scrolling Reaper's Presets combo box from the keyboard is kinda messed up and can be laggy with resource intensive FX (most notably sample-based instruments).

Note: RPL files can only support up to 512 presets. In situations where the total number might hit that upper limit, we save a few spots by doing the navigation slightly differently. We still prepend a number for main folders and a Dash symbol "-" for subfolders, but with this approach, we use the topmost preset in the folder instead of reverting to default settings.

note2: Depending on how the plug-in/instrument exposes parameters you might find that the more presets you add, the slower the FX window loads. That performance hit is unavoidable right now. 

3. Once all presets have been resaved in Reaper's native format, export the RPL file using the "Export preset library (.rpl)" option, which again, is found in the context menu when you hit the "Preset and Performance Management" button in the FX window (and again, that's labeled with a Plus "+" symbol for the helpful sightling).

Once you've got an RPL file to share (or if you need help along the way), put a shout out on one of the community channels. Dropbox admins are usually pretty quick at picking up on any offers to share, but if your offer gets missed, feel free to email scottchesworth@gmail.com.

From here on in, we're going to provide some tips on editing an RPL file manually. This is useful when you can't or don't want to save fake categories in situ and would prefer to insert them afterward, but is probably only gonna be easy to grasp if you've got some experience with editing code. Note that RPL is a fragile format under the hood, so it's wise to take a backup of the RPL in its working state before you start hacking in extras. You will need to be careful to preserve indentation while editing the code, even a single missed space will break the indentation.
Please make time to test any RPL files that you've edited by hand before uploading them into the community. A buggy RPL can take the entire DAW down, and nobody likes a crash mid-flow, yo.

Ok, here come the tricks:

A: Open the exported RPL file in a code editor. Scott and RJH like NotePad++, but any clean editor will do).

B: Move to the second line of the file. You'll find that this is the start of the preset you saved with the plug-in in its default state all the way back in the previous set of tips (before you embarked upon this heady mission to reorganize by-hand, you crazy diamond).
The code will look something like this:
  <PRESET `0 Default Settings`
Read that through character by character. You'll find that there are two spaces at the start of the line because RPL format uses indentation.
Below that you'll find a bunch of code that captures the settings, with each line indented by four spaces. No idea what language this code is in, but it doesn't seem to be human readable.
Below the splurge of machine-speak, you'll find this line which signifies the end of the preset:
  >
Again, note the level of indentation matches the start of the preset (two spaces).

C: Now for some trickery specific to Notepad++, with humongous thanks going to RJH for these productivity tips.
RJH writes:
We now fold the text so that the data is hidden and we can deal with only too lines per preset.
- Change the language from normal to e.g. Python so that the indentation works as block layer.
- Press alt+2 to collapse to the second layer. It should look like this:
<REAPER_PRESET_LIBRARY "VST: FIN-RETRO (UJAM)"
  <PRESET `3AM`
  >
  <PRESET `40 Experience`
  >
 Now, you can select the presets more easily and  move them around, duplicate entries and rename them (to create a category header).
Other useful commands for reorganizing the presets in Notepad++:
Shift+Up/Down to select lines
Control+L to delete lines
Control+D to duplicate the selection
Control+Shift+Up/Down to move the selection around
Alt+Shift+2 will expand the layer again.
Don't forget to switch back to normal text  and to save under a new name.

And lastly, some tips on where Reaper stores presets, in case you need to hose what you've currently got displayed in your Presets combo box for testing changes:
In your Reaper resource path, go to the Presets folder. IN there, you'll find an ini file for each VST or VST3 plug-in, prepended by the plug-in type and a Dash symbol. Deleting or renaming the ini file pertaining to the plug-in you're working on then restarting Reaper to flush its cache will get you back to a clean Presets combo box for that plug-in.

Last update: 16th Feb 2023 by Scott