This is a nicely concise 5 minute video by Tanner Smith (after you get past the fishing section) on five key flies to fish in April.  It was designed for the Colorado river, but baetis are baetis and their recommendations ring true for here in California.  If you want to get right to the meat advance the video to the 3:40 point and get to their recommendations.  Pretty standard patterns though all stages.

https://midcurrent.com/videos/five-files-for-april-on-the-colorado-river/?utm_source=MidCurrent%2BFly%2BFishing%2BEmail%2BNewsletter&utm_campaign=9cf030632b-EMAIL_CAMPAIGN_2019_04_12_04_VIDEOS_AB&utm_medium=email&utm_term=0_8efbf3b958-9cf030632b-42789169&mc_cid=9cf030632b&mc_eid=9ff7f7ed4c