Solder ring fittings are better in less experienced hands, but the secret to
end-feed fittings
is quite simple: clean not only the pipe with wire-wool, but also the inside
of the fitting too,
then flux everything. You can get away with not cleaning the inside of a
solder ring fitting because there is a naturally tinned ring where the
solder is located, so that ring will always be well wetted
by solder and make a good joint.

NB; Thre are different fluxes available, there is one in a beige round tin
called Tempus or something
like that, 'the mild flux'. It is OK but needs all surfaces to be well
cleaned. On the other hand, an aggressive flux like 'Everflux' will make a
joint without having to clean the joints at all! Some people dislike
agressive fluxes because of their corrosive natures.

Andy.