That is so tough if not impossible. It is good that you are aware and want to do all you can.
A few thoughts as a musician who has done what I can even in my own house.
1) Starting soundproofing is to locate your weak spots, whether that’s your walls, doors, or windows.
2) Using heavy books on multiple bookshelves is a fairly effective way to create a sound barrier
3) Plush carpet, rugs, and sound dampening pads can help absorb sound, along with wall hangings, upholstered furniture, and even plants.
4) You can create a soundproof room by installing wall panels or installing soundproof curtains.
5) While not technically soundproofing, white noise is broad-band noise signals played as a constant.
6) If you are able to, replace your hollow door with a solid, heavy-set door. If that’s not affordable or allowed your best bet is to purchase an acoustic door seal kit and attach it to your current door.
7) Soundproofing window inserts provide window insulation that both reduces and absorbs sound. Installing window inserts is an easy and effective way of soundproofing and won’t cause any damage. I used a product called Indow Aucoustics Inserts.