I am an oldie that does the D. Mail suduko every day and I find that it is best to start using the across sections fiirst, find a line wher 2 sections have the same number and then try to work on the remaining section, it works for me, Good Luck
I usually start in whichever 9-number square has the most numbers filled in already - that gives me a good start with what's left. I fill in all of that square then spread out.