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Get some people down there with shovels and carts, same as any public works project earlier than ~1720.

If you have access there is a fascinating program on BBC iPlayer about digging the Victoria Line in the sixties. It's not all manual work but a surprising amount still is.

https://www.bbc.co.uk/iplayer/episode/p00sc29t/how-they-dug-...

There was already a big coal mining industry so I assume they'd have used those techniques.

>How did they build those deep tunnels before the invention of TBMs?

Same approach as TBMs just manual AF. Dig a bit, put in supporting structure. Rinse and repeat.

They've been at it since 1890 though so that helps

The scenario suitable for TBM is surprisingly limited so even nowadays many tunnels are still dig using the good’o way, mostly with explosives

??? explosives? in mud? (well, clay)?!

tunnels are bored depending on the soil, length, diameter. Most projects I have seen use TBMs and the New Austrian Tunneling Method. Explosives are quite the minority (not many tunnels are in solid rock), even the Gotthard tunnels were dug with TBMs