Please make sure to read everything carefully.  And as always, before doing anything, first of all make a new save in the game so that you can go back to that should you have to re-install the original mod when something went wrong.

Also, before converting an esp to esl, check where the esp was sitting in NMM's load order, so that you can place the esp back in its original spot after having to go back to the esp.  When having tons of mods, LOOT often keeps the latest added esp plugins at the bottom of the pack.  I like to keep the lot where LOOT originally placed them.

This is not a mod as such, in that it doesn't add anything to the game.  This is just a simple and small tutorial.  Simple because it takes just a few clicks.  Small because there's not much to it.

The only thing you do need in this case is the Creation Kit.  And some patience.  Plus, some common sense.  The former can be obtained through checking Creation Kit dot com.  The latter two I cannot provide.

Once you've installed the Creation Kit, here's the steps to follow:

Step One:
- Open the CK, hit "File" in the top toolbar and select "Data" from the roll-out menu. Wait for the CK to finish.

Step Two:
- In the Data menu that opened in a small window, scroll through the list with plugins to the one you want to convert, select it and hit "Set as Active File".  The selected esp will get its box ticked in the Data menu list.  Click "OK" and wait for the action to finish  (This is the "patience" part, depending on how fast your computer is).

Tip:
Keep NMM open on its Plugins tab and also keep the Fallout 4/Data folder open.  Then you can activate the esl in NMM and in one go immediately get rid of the converted esp from the data folder.  Keeps you from getting headaches when having converted ten or more plugins in a row and forgot which ones.

Once the CK is done, an at first alarming looking "Warnings" menu will pop up, telling you there's a whole bunch of errors (471, to be precise).  You can safely ignore these warnings and continue on.

Step Three:
- Hit "File" again in the top toolbar and now select "Compact File Forms IDs" from the pop-up menu and then select "Yes" in the Confirm Box* that pops up. A "Done" message will appear with the active file form's I.D. number.  Just click "OK".

*Attention!

As is mentioned in the Confirm Box, the conversion changes the Form IDs that were in the original esp.  As such, the game will see the esl as a new file and mods that place objects like weapons into the world and that you have retrieved will no longer be in your inventory and will have to be retrieved again.  As far as I've seen, mods that add clothing, armour and some such that can be crafted or that add objects to the Workshop, won't disappear and/or go missing.  If such a thing should still occur and it would have a serious impact on your settlement (like half your buildings having gone), then it's of course better to get rid of the esl and re-install the original esp.

Step Four:
- Hit "File" again and now select "Convert Active File to Light Master" from the pop-up menu.  In the confirm box that pops up, hit "OK" and you are done.

Step Five:
- Open your Mod Manager, and activate the new esl file, then open your game's data folder and throw the esp of the mod you just converted in the waste bin.

Voilà.  Easy as pie.  However, please do read and heed the warning that is next.

Warning!

Don't try to...
- ...convert an esp from avery large mod.  With very large I mean a mod that adds a ton of items.  Esl files are for smaller mods.
- ...convert an esp that has other files depending on it or that has several masters.
- Don't convert an esp that needs to be loaded AFTER another esp.  An esl plugin will be added above all other esp plugs and can screw-up your game if that esl was needed to be loaded after another esp.

- Close the CK after every conversion and re-open it to do a new one.  Why? Because sometimes, when not closing the CK, the "Fallout4.esm" is set as an active file in the Data menu.  Double-clicking the file will untick its box, but when then setting the plugin you want to convert as active file and convert it, somehow the Fallout4.esm stayed active in the background and some changes are added to it.  This will result in your game crashing during the load screen. So, be warned and close the CK after every conversion.

That's it.  Happy converting.

This text is what's in the zip-file, because, hey, I had to upload something.  You can as well copy/past this text.

Updated the downloadable file to include the text changes.