Chapter 1: Creation.
Here, you must first determine the amount of letters you want the others to find, or a theme, or miscellaneous (such as "Find an 8-letter word without any prefixesnor suffixes, an example of this is "estimate", or something else completely.)
You also have to determine how you would make it, and this usually goes twofold. Either determine the word (s) you want to insert, or mash random letters in "find long words" puzzles and hope for the best. This is an example of "insert the word" puzzle, (the word DIFFICULTY is obviously inserted)
and this is an example of "mash letters" puzzle.
(I just mashed random common letters and called it a puzzle.)
But, in order to create the best puzzles, you have to strike a balance, and this is how you do it for "Find long word" puzzles.
First of all, start off with a base, I highly recommend using https://www.dcode.fr/boggle-solver-any-size for creation, as you can edit letter placement more easily, and the lexicon is more coherent. I either recommend using the DCODE Common Dictionary or NASPA word list. Either way, start off with a base, and when one letter is not used in the longest words (let's say 8 letter words), try it with a blank and replace that with a letter that forms a word you like. Also, if there's a word that is very obscure, try to remove it if your solutions are mainly those, you will probably find the letter that accounts for most of these words, and try the blank strategy again. If you keep recursively doing this, you can get something like... this.
If you run a solver, it will tell you the bottom E is useless for the longest words, but that is fine, unless half of the board is useless, but who cares, if it's fun it's all good. Feel free to disagree to my views.
Okay, now here's something actually helpful. Personally, if you cram all 16 squares with really common letters, puzzles get repetitive (I made puzzles like this and I have seen the word SERENADE being a solution 3 times), I recommend you this way to start making a base. Bonus story here I just hopped on Reddit and I got recommmended a subreddit about K-pop, and serenade is a kpop song wth hnoadhgoaserhjgfoigf.
This section is a guide to create a "base" position to start with, as mentioned above the common letters strategy can get repetitive very fast.
here are some letter "points" to get off first.
1 point: E, A, S.
2 points: I, N, R, T
3 points: O, C, D, L
4 points: M, P, U, H
5 points: F, B, Y, G
6 points: K, V, W, X?
10 points: Z, Q, J.
I recommend you a dual-point system to generate an initial grid and polish from there, you may edit the numbers slightly, and the grids don't have to match their exact points, it can be a point or 2 off.
How this works is that the point of the letter in a square is roughly the sum of the row and column it belongs, and it makes quite nice puzzles in my opinion. You can always insert a rare letter anywhere, polishing is completely your taste.
The white 1 there means you probably should insert a common letter there, if not that square is surrounded with rare letters and will barely be used.
3
2
1
1
3
1
2
0
0
An example of a puzzle created by this system is this.
It's pretty far-out from the system, it's just a rough inspiration.
Now, this is the hardest part, Vowel-Consonent distribution. I mean it's not hard, but you can get real garbage if you neglect it. Let me give you an example of what can happen if you ignore it, even if you only use fairly common letters. But unless you just make the center consonents, this should not be a major problem.
This will continue.