Once you get the material burning/smoking you'll need to lightly blow on it to get a ember going then you'll need some fine thread, toilet paper, ect... something easy to catch fire and put it over top of that ember and lightly blow until it catches. It is very hard to get a piece of wood to catch fire by magnifying glass alone so this is your best bet.

Dcast explained that well. Practice that a bit and you'll be successful (more quickly) than you were on first attempt. I have a lens about the size of a silver dollar in my kit. With patience, it works well if you start with small tinder and work up to progressively larger leaves, twigs. An abandoned bird's nest or small shavings you whittle from dry wood with a pocket or belt knife could get you started.

Don't know that I can add a thing to these excellent answers other than my pleasure that the subject came up. Great to see the bushcraft skills being passed on.

I've got a different method. Ust a 9 volt pattery and some 4/0 steel wool. Touch the battery to some "fluffed up" steel wool. You'll get the idea in a hurry.

You have to get the spot on the fire tender to the smallest size you can to concentrate the heat.
Experiment with moving the glass closer to the tender.
Good Luck

When you happen to have only what's at hand, like a magnifying glass and the trees. . . The dead and decayed wood that is almost turning to dust will raise an ember quick. That part of the still standing tree that is shaded from the rain, very dry, and decomposed. Don't ask how I found out.

play with it so the magnifier is facing directly to the sun and move it towards and away from the sun to focus the light to a small dot. try lighting soft fluffy substances such as paper towels or cottonballs