Anyone who has flashed a Sonoff device has probably soldered on header pins. It's not too much of a problem if the holes are empty (as shown in the pic on the left).

And although awkward, it is sometimes possible to make a temporary connection using a long-pin header socket with the legs inserted through the holes and pressed sideways to make contact.

It's not so easy with solid pads, or holes blocked with solder as on the S55 etc shown on the right,

Removing solder from blocked holes is often much easier said than done.

But solid pads need wires soldered directly to the pads for flashing, then removed again afterwards.

So here is a cheap and simple trick for making good quick temporary electrical contact for flashing.

Spring-loaded pogo pins are designed specifically for making quick temporary electrical connections, and they can be bought by the hundred for under a fiver, then simply pressed up into the long-pin header sockets.

Such a spring-loaded pogo-pin jig makes good electrical contact and has a firm non-slip grip which can absorb the bumps and dips of an uneven surface.

Be aware that they are available with various tip shapes and sizes, and various shaft diameters.

I chose 1.5mm dia. conical heads because they positively locate into holes without slipping through.

The sharp conical point also digs into solid pads or blocked holes to provide firm grip and good electrical contact.

The shafts need to be thin enough to be pushed into the long-pin header sockets - mine are 1.3mm dia, which firmly locate into some of my long-pin headers, but are a bit too tight for others - but they are affordable enough to get a couple of different sizes to be on the safe side.

Tip: To assemble, hold the header pins upright on a hard flat surface (table) while pushing the pogo pins down into the sockets from above (using thin nosed pliers) to prevent the pin being pushed out of the plastic.

Connect a UART using some dupont cables, press the gpio0 button while placing the jig on the contacts, hold the jig firmly down so all pins maintain good contact, then start flashing.