First components to be soldered on the board should be the ones that are shorter in height.

Start by the resitors.

Then add the diodes

And solder the P2 Jacks

First the ceramic and polyester capacitors

Followed by the electrolytics

Add some pin headers, specially the Power Supply header

Plug the PSU to the board and test for the correct voltage (5V) at all IC sockets.

Solder the Crystal and insert U4

Power the board and check for 6.5MHz signal at pin 3 of U4

The frequency divider is It is one of the few pieces of ZX81 video generating  circuit that run standalone (mostly).

Insert U5, U6, U7, U23 and place a jumper wire from pin 2 of U23 to ground.

Then check for:

3.25MHz at pin 1 of U6

A Negative going pulse of 4.6us (Hsync) at pin 6 of U4 . The HSync frequency shall tick at 15.7Hz.