On your local Mathnasium site, click the Schedule Tab at the top. The click "Schedule Mathnasium@Home Session".

After that, click on a day and a time to schedule your first appointment (you can schedule more later). Then you'll be prompted to create login. Pick which option is best for you and then click "Next". You should remember this login since you'll need it later to schedule lesson in the future.
Next, you'll be prompted to enter your student's name. Please enter their first and last name. If you have multiple students having lessons at the same time then list them all like "Alex/Aric/Kyle Guppy". If your students are not having lessons at the same time, then just schedule separate lessons for them.
Finally, you'll be prompted to enter the Number of Students. It's easy to miss on a desktop, so there is a picture below for your reference. Parents who have multiple children having lessons at the same time must choose accordingly.

Feel free to play around with the functions. You can set up recurring lessons and do other things as well!