After logging in to iLab, you may see a screen like below.
Click list all cores on the left menu to find the core you plan to use.
After selecting the core, you should be on a page with a Schedule Equipment tab which you should select.
On a page of equipment, click the view schedule button for the instrument you plan to use.
Click on the time your planned use starts and drag down to select the length of your reservation.
On the final page, you can adjust the time of your reservation, select a PTAEO(s) for payment, or cancel the reservation.