Cross on the screen changing into a visual stimuli after a given delay

If this template helps then use it. If not then just delete and start from scratch.

MacOS
PsychoPy 3.2.3
Standard Standalone

In my experiment, there are three blocks that are going to appear randomly. In each block, there will be a cross on the screen that disappears for either 300ms, 500ms or 700ms. After the delay, one of three visual stimuli appears on the screen. Each stimulus needs to appear after each delay 3 times in one block (which gives me 3 blocks of 3x3x3: 81 trials in each block).

It is my first experiment using PsychoPy and when I try to read articles or watch videos but everything sounds as if it was written in a foreign language. I can not understand how to make an excel file that will work with it, how to make that loop within a loop to randomize it, how to add the visual stimuli and the cross on the screen to it… Basically nothing. All I know is how to add instructions for participants.

I would be extremely thankful if someone could provide me with a starting point.