As the bottom rail will be a through mortise and tenon, you will need to transfer these gauge lines on the opposite face too. Do this first using a pencil and run the gauge lines between these pencil marks, then use a knife and a square to score across the mortise hole ends. To make sure this is transferred accurately, use the knife to create nicks on the corners and follow these round with the square. Repeat this on the opposite side.
When chopping using your chisel, make sure you start off with a knife wall as you don’t want to break off the end. Then only take small chops at first in this area as the amount of material you are leaving in place is quite small and fragile.
You only need to go down ¼“ (6mm) deep for this mortise as it is a stubbed tenon which means it doesn’t go all the way through
Use a thinner chisel to lever out the waste wood.
Chop across the mortise hole, when you get to the knife line, turn the chisel around to make sure not to bruise the good wood. Lever out the waste wood and pare cut along the bottom until you have an even recess. Set your square to ¼” (6mm) and check the depth, once you have achieved the right depth, start chopping your next mortise for the bottom rail.
When chopping from the other side of the through mortise watch out for the shallow mortise hole as bursting through in an uncontrolled way can result in breaking out the edges of the mortise. Listen to the sound of the chop change as you get closer to the middle.