Alright, here's some code. It's not quite fully functional, but it has the basics of each step. You'll probably need to separate out each chunk of put it where it belongs. I'm not sure about how to use SDL, so you're kind of on your own there (unless someone else can comment on that). There could be some errors in this code sample, but it should help illustrate the idea. You could also safely make some alterations to this, but I'll leave that up to you. I went for simplicity in this small sample.

// I'm assuming you've already opened the video...
SwsContext* imageConvertContext = NULL;
int frameFinished = 0;
int videoStreamIndex = 0; // I'm making this up, of course
char* buffer;
AVPacket packet;
AVFrame* decodingFrame;
AVFrame* rgbFrame;

buffer = (char*)malloc(codecContext->width * codecContext->height);
decodingFrame = avcodec_alloc_frame();
rgbFrame = avcodec_alloc_frame();
av_init_packet(&packet);

// Use PIX_FMT_RGBA instead if you need rgba...
if (avpicture_alloc((AVPicture*)rgbFrame, PIX_FMT_RGB24, codecContext->width, codecContext->height) != 0)
{
// error
}

imageConvertContext = sws_getCachedContext(NULL, // current swscontext
codecContext->width, // source width
codecContext->height, // source height
codecContext->pix_fmt, // source format
codecContext->width, // dest width
codecContext->height, // dest height
PIX_FMT_RGB24, // dest format
SWS_FAST_BILINEAR, // flags
NULL, // source filter
NULL, // dest filter
NULL); // params

if (m_imageConvertContext == NULL)
{
// error...
}

// This is where the decoding and converting part go. I'll assume you've got a decoding loop already.
// This isn't quite a proper loop, but it has the important part of using sws_scale()
av_free_packet(&packet);
while (av_read_frame(formatContext, &packet) == 0)
{
if (packet.stream_index == videoStreamIndex)
{
avcodec_decode_video2(codecContext, decodingFrame, &frameFinished, &packet);
if (frameFinished)
{
sws_scale(imageConvertContext,
decodingFrame->data,
decodingFrame->linesize,
0,
decodingFrame->height,
rgbFrame->data,
rgbFrame->linesize);
break;
}

}

av_free_packet(&packet);
}

// Now copy the rgbFrame into the buffer
for (int i = 0; i < codecContext->height; ++i)
{
memcpy(buffer + i * codecContext->width, rgbFrame->data[0] + i * rgbFrame->linesize[0], codecContext->width * 3); // change the 3 to a 4 if doing rgba
}
SDL_CreateRGBSurfaceFrom(buffer, ...);

// Clean up when you're done... probably at the end of your program
sws_freeContext(imageConvertContext);
avpicture_free((AVPicture*)rgbFrame);
av_free(rgbFrame);
av_free(decodingFrame);