General guide:
As I didn't bother translating the tutorial screens or some of the fancy buttons (though all untranslated buttons have tool-tips that describe what they do), I'll provide a small (edit: uh, not so small) getting started guide.
After the intro, the first thing you want to do is to summon some girls. Lower-right, click the green button with circling arrows. Note how it shows a number, these are your summoning points. It cost 1 point to summon a girl.
After summoning a girl, you then decide what to do with her. Heart button to bring her into the harem. Jail button to send her to the dungeon. Or if you don't want her, you can ignore her and fool around with your other girls (she'll eventually disappear) or you can immediately summon a new one.
Once you've got some girls in your harem, it's time for training. Select a girl by clicking on her name (or from list view, button on mid-right). Now you can see her stats again, but what you really want is to click the red/green Play button to take her to the bedroom.
In the bedroom, your main concern is the four tabs on the left side of the screen. Have a look through the options, but eventually you'll want to choose a sex action. (Avoid the service options, you want the ones where atleast she gets some pleasure.) Adjust the "Speed" slider on the right to get things going.
Now have a look at the two bars on the bottom. When "Arousal" reaches 100, the girl will orgasm and you get a big XP bonus. (Getting her to orgasm as much as possible is the main thing you want to do in training.) "HP" lowers as you do various (but not all) things. Performing an action that hurts the girl (e.g., sex when her vagina/anus is undeveloped) will drain HP much faster. Her orgasms will also deal a big HP hit. Finally, there a some sub-actions you trigger with right-click that trade a middling HP hit for an Arousal boost. Your task in training is to perform sex actions as long as you can (with as many orgasms as you can manage along the way) before her HP runs out.
When her HP does run out, the scene automatically ends and you collect your XP. This levels both MC and the girl. Girls' levels give them more HP (so longer training) as well as boosts to combat stats (relevant later). MC's levels give various upgrades (MP, harem and prison slots) as well as unlocking spell.
When you do eventually level up, you'll get your first Spell "Gaen", a healing spell. Healing will be your bread-and-butter magic during training allowing you to extend the sessions
much longer. When you MP is low after a session, you can either wait for it to slow recharge or pay Summoning Points for an instant recharge (button is upper left next to MP bar).
And that's how things will go for a while, summoning girls and training them up. But eventually, randomly, when you try summoning a girl you'll get a Hero Invasion instead. These enemies fight your harem and can kill lovers (low-level lovers are typically one shot). If your entire harem is killed, it's game over. The mechanics are pretty straight-forward, so I won't waste time on that. However, you probably will eventually wonder why you have non-healing spell options. These are all debuffs that alter the Hero's body to lower her attack and defense. (Don't worry, with enough MP you can alter her back to her original form afterwards.) Assuming you manage to whittle down her HP and defeat her, the hero will be imprisoned in your dungeon.
The button to take you to your dungeon is on the mid-right side. You'll find the mechanics here similar to regular training, so I'll just talk about the differences. First off, hero's start of in a brainwashed state. (Sorry, I had to abbreviated the translation as "Brnwshd" to help stop the game from clipping off names). While in this state, her Affection is at rock bottom (naturally) and she is stuck in prison (and while in prison, her Affection cannot be trained up, though the other three stats can be raised). To break her conditioning, you have to inflict unbearable pleasure on her. Specifically, you use the edging mode (pinkish button, lower-right corner). When edging mode is on, she won't be allowed to orgasm when you bring her Arousal to 100%, and when repeated will eventually break her out of her brainwashed state. When this state is gone, she can then be invited into the harem.
And that's basically the game. Keep training girls, raising their affection which also raises their race's Friendship score. When you get all five to 100%, this triggers the ending. You can see a summary of Friendship scores on the Options screen (gear icon, upper-right).
Warning: Clothing items only enter your inventory when a girl enters your harem (or jail, I think). If you summon a girl with rare clothing, make sure you bring her in. If you don't want her, you can dismiss her afterwards.
Tips/spoilers: