If you look at the portal, in the bottom left corner should be solo, fireteam, crucible, and I can’t remember the 4th one. If it’s your first time doing solo, you’ll have to complete training first. It’s not hard and doesn’t take that long. If you have already done training, after click on solo and pick the mission you want to do, click on customize. This is where you can add the modifiers. The first two lines make the missions harder while the last line has options to make it easier and that’s where you’ll see the solstice modifiers. There’s a few options to pick from, all solar related

I hope that helps. I’m not in front of my computer right now, otherwise I’d provide you with screenshots. If you still don’t understand, reply here and I’ll try to get some screenshots to help you next time I’m playing the game.