Reddit mentions of 4 Pack Active Basic Women's Basic Tank Top (1X,White/Charcoal/Black/H Gray)

For myself, it honestly took trial and error, so I'll spare you the messy beginning and tell you where to start.

I'd strongly suggest getting this product. White, light grey, dark grey, and black spaghetti strap tank tops, no built-in bras, nice long length. Black/white monochromatic is imo essential to start.

Buy jeans in grey, black, and dark-wash blue. I personally don't like light-wash blue or tan, but if you do, then do you. Don't get busy with accents on your pants, and don't try for colored pants quite yet. You just want something basic that fits and is flexible.

Finally, find cardigans you like. Try to find styles meant to be worn with an open front, so you can pair with the tank top bottom layer for color experimentation. Again, initially, stick to monochromatics--find something black, find something grey, maybe something white if you're not a particularly messy person.

Then, just buy stuff and try it on. I started out buying jewel tones in purple, green, and blue; I quickly realized it just didn't work well for me, and I was better with earthy tones. Buy colors you wouldn't otherwise try and try them. Be realistic about the colors that don't work on you. Narrow down what works to three colors, then aggressively search for what matches those.

Target has a very nice selection of tank tops and T shirts for relatively cheap. They also have button-up cardigans, although I'm not so much a fan of those. Amazon has multiple listings like this, which help a great deal in mixing and matching. Always go shopping with a plan, though--wear at least two of the three items, and shop for the third. Don't let yourself get distracted by something that's theoretically nice--if it doesn't fit what you've already built, then it's never going to work.