An oversized T-shirt does most of the work on its own. The mistake is trying to add to it. Nearly everything I could tell you about styling one is really about leaving things alone.
Start with the trousers, because that is the decision that makes or breaks the silhouette. A boxy top sits best over a relaxed or straight leg. Black trousers are the easiest answer and they work almost every time. Washed denim looks right with a cream tee because the warmth of the off-white softens the blue. Grey, olive and brown all sit well too. With very slim trousers I personally find the proportions harder to balance, because the top ends up carrying all the volume on its own. If a slim leg is your look, wear it your way. I am describing what works most easily, not writing a law.
On colour, it helps to know what you are actually working with. We call our colour seedpearl. In plain terms it is a warm off-white, the shade most people would type into a search bar as cream. Warm tones like that sit best next to calm colours. Black, grey, faded blue, olive, deep brown. Keep the outfit to two or three quiet colours and the tee reads as considered without anything else going on. Loud patterns and bright colours right next to it tend to compete with it, and then nothing in the outfit gets to lead.
If it is cooler, layer with something open. An unbuttoned overshirt or a straight jacket sits well over heavy cotton, and a substantial tee does not disappear under it. The print on the back gets covered, and that is fine. It was never meant to be on display at all times. Wear the layer and forget about it.
Shoes, keep them simple. Trainers you actually wear, not ones you are worried about creasing. Boots work when the trousers are heavier. If the shoes are the loudest thing in the outfit, the tee stops being the point.
The one thing I would genuinely call a rule is fit. Get the size right and everything above gets easier. The size guide lists the garment measurements flat, chest, length, shoulder and sleeve, and the method is simple. Measure a T-shirt you already wear and compare. Every size of DROP 001 is cut oversized, so you are choosing how much room you want, not whether it is roomy.
Then wear it the way it was cut. It falls straight to the hem, so I leave it untucked. If the sleeves sit long, push them up once and stop adjusting.
The simplest version of all of this is the tee, black or washed grey trousers, simple trainers and a jacket when the weather turns. It is not a formula. It is just what keeps the garment as the point of the outfit, and it works in almost any room a T-shirt belongs in.
The goal is not to look styled. It is to look like you got dressed once, got it right, and went about your day. An oversized tee in a calm colour does that with very little help. Give it decent trousers and simple shoes and let it work.