A friend has a garage and the adjoining garden covers the lower 1/3 of the
left-hand wall.

Water comes in from the soil through the wall. Whatever waterproofing was
there is not working any more. The internal plaster is blown and there's
puddles on the floor.

Getting to the outside of the wall is not an option so I've been reading
about tanking. Seems like there are a few gotchas although the sites I've
read are all companies that can do the work...

Is it best to get a professional in to determine exactly what is required
and to do the work, or is tanking a relatively easy thing to get right?
I'm happy putting some effort in to do the job but would rather not if it
is too easy to make mistakes that mean the fix only lasts a short time.