One of the things I have always done for each media I ever worked with, is making a colour chart. It is a lengthy process when done properly but it is soooooo worth it!
Firstly it greatly helps getting a "feel" of what the medium is like, how different colours work with others, how strong the pigmentation is, etc.
As I have recently started working with oils, it was time to spend a weekend making charts :) I divided A4 sheets of Arches paper for oils in a selection of squares which I taped off using cheap washi tape. It has the benefit of coming off easily without ripping the paper and leaving super smooth edges (well.. mostly). It's also just about half a centimeter wide which is perfect for this exercise. You can obviously use larger formats as well but I find this the right size to have enough paint in one square to see the colour and to have enough space for a variety of colours to mix with. I then work systematically through all my colours but you can obviously opt to only use the ones you mostly use.
I have 1 column on the left which is dedicated to the colour this card is for. In the example above: Ultramarine blue. I then add a bit of white to the pure ultramarine and a bit more each time until I have 5 shades of the ultramarine. In the top row, I add the pure, straight out of the tube colours I want to mix with the Ultramarine. Here a selection of earth colours. The second row is pure ultramarine with any of the pure earth colours mixed.
The following rows are each mixed with various grades of white. After a while, it becomes second nature to guess which colour combination makes which shade but this exercise greatly helps to jump start the process. And you obviously write down the colours! Here are a few more examples:
Lemon yellow with the various blues I have + 1 section with various gradients of black
King's blue light + earth colours
Cobalt blue + earth colours