My uro had me pull it along the leg, mainly to give a stable target with a backstop as it were. Makes sense.
He has me inject near the base, at about 9 & 3 positions, yes, straight in (right angle). It is a bit problematic for me, seems that there are nerves that run at exactly 9 & 3 - hit one of those and you'll never use that spot again (been there, done that). Also I seem to have more blood veins than most, so the clear spots are limited. Any angle that would put the needle in vicinity ot the urethra would be an issue.
Alternating sides, and limiting use with at least a day in between shots are his suggestion to avoid scarring. That left-handed shot for a right-hander is a challege.
He also uses the "pull back on the plunger to get a couple of drops of blood" approach. That way you know that the med is going to the right place.