We try to do it every day. We were makpid with all of ours children, from just about day 1.
How I do it? I place infant on my lap, criss-cross his/her hands and hold them together with one hand in correct position, so I generally aim right on. We’re not so fussy about opening her fists if she’d make one, though.
Problem is when they get older, they always want me to wash them “the funny way”!