In article , R
writes
I have seen advice here and there about 'pinching out' the growing tip
of some plants (e.g. tomatoes and peppers) to encourage lots of fruit.

I assume that 'pinching' out means 'cutting off'

No - it means pinching out ;-)
Look at the end of the shoot, find the tiny bud/pair of leaves right at
the end of the shoot, and pinch them out between finger and thumbnail.

and if so,

a) how should it be done (e.g. would a pair of scissors do? should it
be straight or at an angle? does the end have to be treated to prevent
infection?

You are pinching out right at the very end. No need for treatment, and
it's so small that straight or at an angle makes no difference. If
anything, it's a curved cut following the shape of your thumbnail ;-)

b) when should it be done? Is it when the first fruit appears?

When the 4th truss has set is the advice I've read.

c) what plants other than peppers and tomatoes does this apply to?

Fuchsias. Coleus.
Anything which you want to make bush out

--
Kay Easton

Edward's earthworm page:
http://www.scarboro.demon.co.uk/edward/index.htm