Reading a Desi calendar is simple once you understand three things: the month names, the year number, and how a Desi month sits inside a Gregorian month.
The twelve months
The Desi year has twelve months in a fixed order: Chet, Vaisakh, Jeth, Harh, Sawan, Bhadon, Assu, Katak, Maghar, Poh, Magh, Phaggan.
Each month begins on a fixed Gregorian date, so the calendar for a year looks tidy. For example Sawan always starts on 18 July.
What the numbers mean
A cell on the calendar shows the Gregorian day number and the Desi date below it. The day number in the middle is what you would use in everyday life, and the Desi date underneath tells you which day of which Desi month it is.
On this website the Desi calendar shows both dates together for every day, with today highlighted.