To insert SIMs into a Solo+, first move the rubber door out of your way.
There is an orientation diagram to show the orientation of the sims. The two SIMs actually face each other, if you are looking at the copper connectors. The notch goes down first, into the sim slot. See the images below.
The sims then click into place, and may require a small tool to click all the way down (a pen, pencil, etc.)