Using Midjourney is relatively simple.
Here's the official Quick Start Guide
Naturally, as the images are only 1MP (1024x1024 pixel in square format) you will want to upscale them if you want to print them in any meaningful format.
The best way of doing so for free and without any watermarks that I have found so far is through the Pixelcut Upscaler.
I've found that generally I can upscale once by 2x and then upload the upscaled image again and upscale it 4x. This way you can get around the upload size limit and still upscale 8x.
For print, my rough rule of thumb is that printing below 250 dpi (dots per inch) won't look too good on finer prints. And as one inch is 2,5 cm, that means we need roughly 100 pixels per cm.
So if we upscale a 1024 x 1024 px square image by 8x, that means we can print it on a maximum size of 80 x 80 cm.
Or go with a different format by adding —ar 64:21 to the end of your prompt.
The prompt for this image was:
/imagine Photo of a colorful liquid abstract, in the style of psychedelic dreamscapes, striated resin veins, yup'ik art, fantastical street, psychedelic overload, cosmic inspiration, intense close – ups —ar 64:21