The Cross Stitch:

It makes a cross by stitching into the lower left hole from the back of the fabric and into the upper right box from the front. So you have a diagonal at first. Repeat this process as long as the pattern allows.
Hold the remaining thread on the backside and make sure that the needle always runs over it, so that you can fix it by tightening the thread. You quickly get a feeling for the thread tension. It is best to loosen up as much as possible and not to pull the thread too much, otherwise you run the risk of warping the Aida fabric and the result could look crooked and crooked.
When you have reached the end of the back row, it goes to the back row. Back you have to stab from the back right down to the front left up. Because in larger embroideries you tend to turn the embroidery to the right, you should pay special attention that the embroidery direction is always the same, i.e. that the thread always points in the same direction: Thus, the leading thread leaning to the right should always be below the return thread leaning to the left. This way vertical stitches will appear on the back. If you do not pay attention to this, the finished embroidery image might look unsteady.

When the first row is finished, the last cross can be closed in one step and the row below it can be stitched immediately.
The Thread change:

When the thread is used up, you sew it together with the beginning of the next thread, which we show here in orange, as you did at the beginning of the work. After about 5 stitches you can cut off the two remaining threads again.

When you have reached the end of the embroidery work, you can sew the last thread by pulling it through some of the vertical stitches on the back. Finally, just cut it off and that's it!

You could now simply tie a knot in the end of the thread as a stopper, thick enough that it won't slip through the hole. This is the variant for the impatient, but it doesn't necessarily last forever. It is better to sew the thread: when you start embroidering, if you stitch the needle through a hole from the back of the fabric, pull it through until there are about 5 cm left on the back. This remaining thread is then embroidered in the following steps. After approx. 5 stitches cut off the remaining thread, so that it does not interfere with the rest of the embroidery.