Beginner: what are "flat" sleeves with a seam?

I’m a beginner knitting a doll cardigan (the “Simple Doll Cardigan” on Ravelry by Xilan Allen with raglan sleeves). When I went in the neighborhood knitting shop, the German woman who works preferred not to sell me two sets of needles to knit this cardigan, as the pattern asks for the sleeves to be knitted on double pointed needles (she said it would make things too pricey and she seemed to doubt I would even finish it…).

She said I should knit the raglan sleeves flat and then join them with a seam along the bottom using a tapestry needle and the yarn. I have found several YouTube tutorials on sleeves, but none where the sleeves are knit flat. I just finished the body so I guess I start the sleeves somehow on the armholes… Can anyone please describe to me how to do this?

Welcome to KH!
Knitting flat means that you would knit back and forth in rows rather than joining to knit in the round. When finished, you would seam the two edges together to make a tube for the sleeve.
Seems easier to do this on dpns as the pattern suggests because you’ll have problems turning at the end of the first several rows. I’m assuming you have sts on hold for the sleeves?
Easier to knit a tube here rather than flat. Purchase a pair of dpns elsewhere if you feel more comfortable doing that.

Cute pattern. I can’t imagine talking a customer out of buying, making a suggestion to do something else, yes, but not insisting. I agree with salmonmac that you’ll want to work in the round.

Wait, what?! Suggestions are fine, but she sounds rude to me. It looks like an easy pattern so I see no point in knitting the sleeves flat.

If you get tired of having to switch to DPN or having circular needles of the same size in various lengths and having to change needles you can look into using one long circular for magic loop or knitting in the round on two circulars. At some point you might want to consider a set of interchangeable needles so that you can have various cable lengths and not have to keep buying new needles. It can be expensive upfront but in the long run saves lots of money.