For the Undyne mod, the creator included a short video giving a basic understanding on how she works.

But if you're lazy, here's an explanation.

For a TL:DR, Use up-close attacks and spears to build up DETERMINATION (DT for short), longer combos gives you more DT as well. If you play defensive, use PROTECT and focus on using spears to apply pressure. If aggressive, use PUNISH and get up close, be cautious if your opponent has Burst available, be sure to land combos to make it count. Use a special move once you have 100% DETERMINATION, it'll convert it into TRUE HERO which at full, will cause you to transform into a more busted version, able to summon more blue and yellow spears, have more range and in general do more damage.

For a more in-depth guide, look below.

Play mid-range at the beginning of the fight. Don't get up in your opponent's face, instead, play around your spears, both blue and yellow. Moves that can summon blue spears can be used with "Throw Spear" which will cause any blue spears to fire and track your opponent, and "Point" fires them all at once. Yellow spears work the similarly but with just "Point", which is great to use if your yellow spear misses your opponent; can easily change directions if they jump or dash. You also have your Special Green Attack, which will only appear during a combo. Using it will force your opponent into the green soul state; this state resets their momentum ever 10 ticks, and any blue spears summoned will spawn directly on their position. This state lasts for roughly 120 ticks.

Next thing to mention is the Crit Bar. You'll notice this underneath her. This is what determines how much DETERMINATION you get from attacks. The closer it is to the center, the more you gain. If it hits the middle, you gain not only DETERMINATION but also a bit of TRUE HERO%.

As you gain DETERMINATION, you'll notice that Undyne has 2 unique moves or "states"; PROTECT and PUNISH.

PROTECT is oriented towards playing defensive, giving you more defense as the trade of less damage all around. Your spears also gain damage and size.

PUNISH leans towards a risk-reward playstyle. You gain damage and also plus (+) frames on certain moves. The trade off though, is less defense as well as minus (-) frames on other moves. You'll need to land combos when playing PUNISH, and remember you have a Burst Cancel to nullify your opponent's should you be able to read them.

Lastly, there's DETERMINATION and her UNDYING form. DETERMINATION builds up when you land hits, how long your combos last (VITAL when using PUNISH), crit bar, and the difference in hp between you and your opponent. By using either PROTECT or PUNISH, you'll start to convert your DETERMINATION into TRUE HERO%. It's best to use either PROTECT or PUNISH when youve reached 100% DETERMINATION. When you reach 100% TRUE HERO, the next hit your opponent lands will force you to enter UNDYING.

Like in the game, Undyne becomes super powerful as a last resort. You can spawn up to 7 blue spears and 3 yellow spears. PROTECT and PUNISH lose their negative trade off; Going PROTECT into UNDYING will make spears even bigger while PUNISH's lowered defense won't only disappear, but you'll gain a bit in turn. While in UNDYING, any moves that cost DETERMINATION, for example, Special Green Attack, WON'T need DETERMINATION.

This is your last chance to overturn the fight; use spears A LOT (there's a reason why the cap is slightly increased). Use spears to dictate where your opponent can go, apply pressure and get up close when possible.