I've tried a couple of times to crack this market but the nearest I got was a reply saying they'd have been interested if they hadn't published something similar. Typical of me to have missed the boat!
You can find a few wise words and guidelines on serial writing for the women's mags on Sue Moorcroft's blog.
Sue is a creative writing tutor for the London School of Journalism, Writers’ News Home Study and University of Leicester and as well as serials, has sold hundreds of stories to the womags.
The most important bit of advice is DON'T write the whole thing first. Just send in a proposal or a brief synopsis and maybe the first few thousand words. That way if they've run something similar, you wont have wasted weeks of work.
If the editor wants to proceed, they'll usually ask for the first installment. Be prepared for a long haul, especially where People's Friend is concerned. A full serial can take up to a year or more to get every installment accepted (they do like to give some input into the plot)and then may not be published for a couple of years after that. The good news is you can sell the thing again as a novella (or as a novel if you can expand it to around 80,000 plus words). There's also the large print market to consider.