My first recommendation for you is to find an organ collection that you like and start playing it one page a day. However, do not attempt to play the entire for-part texture with hands and feet combined right away.
The best way to go about practicing sight-reading is to play solo parts first. This means playing the soprano part separately, the alto part, the tenor and finally the bass part separately in the entire collection.
After you do that, go back at the beginning of the collection and start practicing two-voice combinations. Make sure you are covering every single one of them without missing any of these steps. Every step and combination is just a little bit more difficult than the previous one and leads to the next step.
The following step would be to practice three-voice combinations from the beginning of the collection. Again, try not to skip any combination of three voices and aim for your playing to be fluent and free of mistakes.
This may mean that your practice tempo should be much slower than the concert tempo. This systematic step-by-step approach allows you to progressively advance in sight-reading and be able to play with four parts together later on.
By the way, you can make an experiment which will prove to you how effective this system is. Here is what you have to do.
Before practicing your sight-reading, try to play all parts together of your first piece right away. You will soon discover how difficult it is and you will make many mistakes. Make a mental note of exactely how many mistakes you have made in one page of music.
However, repeat the same exercise after you complete this training and you will discover who much further you have progressed with this systematic step-by-step approach.
If you want to become competent in organ sight-reading, try to practice it every day. If you skip one day without practice, only you will notice it. If you spend two days without practice, your teacher will notice it and if you skip three days without practice, then everybody else will notice it.