(GUYS I'M HONESTLY SORRY FOR USING THIS AS THE OPENING IMAGE THERE'S SO MUCH WRONG WITH IT LET'S GET BACK TO IT LATER ON)
Wa-lolita can simply be defined as a lolita outfit that takes strong influence from kimono and the outfits put together to go with one, kitsuke. What kind is not stated and since this style is both rare and difficult to pull off the parts that are deemed important tend to be undecided and change with time a lot. What's more, a kimono works on a completely different set of aesthetics than lolita, making it even harder to combine the styles.
So what can be considered kimono/kitsuke relevant influence?
1) The cut and basic shape, of course. The front overlapping, kimono sleeves, wide belt. Not the narrow frame though since the lolita part of it tends to want poof, and lots of it.
2) The print of the fabric. This is nowhere near as important as the first one, though, as wa-lolita can use whatever type of a print.
3) Similarities in accessories. If you're going to wear western shoes, wear ones with thick soles. Rocking Horses might work well! In the year 2005 -or so clunky boots were also acceptable, in fact the "official lolita truth" du jour was back then that if you cannot get yourself a pair of geta you need clunky boots. Other accessories you might consider are fans if the weather is hot (you'll thank yourself for one) and handbags - either use a furoshiki as seen in the first photo, a pouch such as seen here, a handbag with a suitable print on it or a cute pouch -type handbag - anything as long as it could be seen with a formal kimono as well.
4) A decent hair-do unless your hair is absolutely amazing by nature. And even if it is, consider putting it up or you're risking an end result like young me in the opening image. Not too flattering, right? Don't be stingy with the hair ornaments, with the amount of poofy and showy stuff going on below thin hair is the last thing ever that you want, it makes your head look disproportionally small.
With the right items you can make almost anything work. Almost any dress that doesn't go strictly against the idea of kimono can work as well, but not in just any coord and especially not if you're trying to use several questionable items. Still, I'm not the real rule maker here because like always, the end result is what defines whether or not you've managed to make a wa-outfit or not. Don't stop from trying things just because I say so because I'm this random chick on a tiny island in the middle of the Atlantic and I don't rule the lolita world. :D
What is enough influence?
Exhibit A. This is not a wa-lolita dress. Sure, it's got a Japanese print but when that is the only kimono influence the dress has it's not enough. There's no sign of the overlapping front or a wide belt to go with it - nothing. I've heard people classify all Japanese print dresses as wa but that would mean that we'd have to define EVERY lolita dress by their print only, and that would be a wee bit frustrating - I mean who'd get the rose prints, the sweet, the goth or the classic?
To make this dress work in a wa coord you'd have to add lots of related accessories and perhaps an obi as well - a pretty hanhaba or a kantan obi would be both affordable and easy enough to wear, esp. kantan, or pre-tied, obi. I've noticed some people mistakenly call it fauxbi, thinking it's a "fake-obi" by the way. However, a kantan obi is just as much of an obi as any of the fancier types. It just happens to be more informal, often worn with yukata for example.
Exhibit B, a little harder to define. Here at least there's a whole outfit and you can see that even though the dress itself is not exactly kimonoesque the accessorizing, wide sleeves, open front, hairpiece etc. add so much to it that I would not immediately rank this as "non-wa-lolita".
Btw check them boots!
Exhibit C. This is already so close that even with the lack of sleeves it could count as a wa-dress. There's that overlapping front that continues from top to the hem, the print, even a wide belt that looks a little bit like an obi.
Exhibit D: definitely wa-lolita. Everything that's needed is there to make the immediate link to kimono. It's even got a lace haori!