The trick to friction shifting from a smaller rear cog to larger cog is to ease off pedaling pressure for just a moment while shifting. Feel for the slight resistance as the chain encounters the side of the larger cog. Resume pedaling pressure and nudge the lever very slightly to complete the shift. Trim as needed.
Practice very deliberately at first. Stop pedaling and coast, shift up from smaller to larger cog. Feel for the resistance. Resuming pedaling gently to complete the shift.
After a little practice it'll become intuitive. You won't need to stop pedaling. It's comparable to learning how to use a clutch with a manual transmission.
The shorter the cable, the better the feel. I find my bike with bar end shifters a bit finicky in friction mode, and mostly I rely on muscle memory of the shift lever throw. On my bike with downtube shifters the feel is more palpable.