Getting a single girl isnt very difficult. Especially if you use the guide posted in this thread. But if you need some tips:
Each girl has one stat she wants:
Mary wants knowledge.
Sarah wants fitness.
Lucy wants charm.
At the start of every day you do the planning. Morning, evening and night. Energy is only regained from resting, not from sleeping at the end of the day. High energy means that you get more stars when you do an action, more stars gives you a bigger stat increase for fewer energy points. So you want to keep your energy near 100%.
During weekdays you can only go to school in the morning (basically a wasted action) and you can raise your stats with the other options.
During the weekend you can also do something useful in the morning, and on saturday you can go shopping, on sunday you can take a girl on a date (after you get a girl as a girlfriend).
The basic moves you make near the start is
Morning: school
Evening: stat increase (pick the one the girl wants)
Night: rest
Each girl has two phases, the seduction phase and the dating phase.
The seduction phase requires you to hit certain stat milestones. For example, to get mary, you have to grind up knowledge. At roughly 20, 40, 60, 80 and 100 you will get an event with a girl if you go to school in the morning. When you have these events pick either the kink choices or love choices (you can see which is which in the guide, or click one and see if a heart, or an upside down heart appears). You know you have seduced a girl if you have sex with her for the first time.
Once you have her a a girlfriend you can date her (on sunday you can select go on date in the morning). During these dates you want to pick positive options to increase her stats further. To max out a girl you need to have 200 in the primary stat+100 in a secondary stat (again, the guide tells you). If you did 4 or 5 events (go to school and take her on dates to trigger them) and take her on 4 dates and pick the good option every time, you will get the ending. Note that some events require you to trigger them: Sarah requires an umbrella, lucy requires you to tutor her.
There are a few extra mechanics:
If you grind out lucy enough you can start skipping school in the morning. As far as I can tell that doesnt do anything negative. In fact, during the morning you can go shopping and do advanced actions.
Second is advanced actions. If you have 2 stats above 50 you can do an advanced variant. For example, knowledge+fitness unlocks martial arts. Martial arts increases both fitness and knowledge. But you can only do these actions in the weekend and during the morning (if you can skip school).
Simple guide for a single girl:
Pick a girl you want (lets say, mary)
During the week days: m
orning: school,
evening: increase skill the girl likes (for mary, knowledge, so you study),
night, rest.
During the weekend, 2 rests+skill increase.
Whenever an event with Mary happens, go to the guide and pick either the love or kinky option. Near the end of winter you should have nearly 200 of the stat the girl wants (for mary that would be knowledge) and have the girl as your girlfriend. At that point, increase the other stat (look in the guide which one you need, with mary as an example, you need fitness for her kink route and charm for her lover route). Every sunday take her on a date, make sure you have enough money.
Harem route:
Didnt really figure this one out yet. But I think the primary idea is:
-go for lucy first to unlock the ability to skip school
-Once you can do whatever you want in the morning, start increasing all your skills to 50.
-Then do the advanced actions that allow you to increase multiple stats at the same time. Make sure you stay near max energy for 3 star actions, make sure you go to school once in a while to advance the girls their events.
-get every girl to the point you are about to see her ending, but then lock her in the girl screen (morning, bottom tab of your diary), and go for the next girl
It seems hard to do since you need to date every girl like 4 times, and you can only do so on sunday. Maybe I misplayed or miscounted and you can do it in 3 dates, but even that is 9 dates in total.
As for working and shopping. I didnt really see the advantage of it. Work seems to cost similar amount of energy, the money you gain is about enough for 1 stat booster items. So whether you work for money, and then put that money into books/clothes/proteins, or you just grind your stats, it seems to give you the same result. Only work so you can buy the umbrella and pay for dates.