Be honest, clear, and detailed: Nice space. Fairly good location - everything seems to be just across the bridges.
Use specific examples when you can- it's one big space with little division between sitting room, bedroom, dining room/kitchen.
Share what surprised or delighted you-the big blue couch/sectional was very comfy.
Include info, good or bad, a future guest should know before booking- bring earplugs. It isn't the fault of the (delightful) host, but the flat has no noise protection from the next door neighbor's alarm (5:30am) or the people hanging out at the curb until all hours (4am). Bring an eye mask too, since the outside light isn't very well hidden. Go to the food market and bring back a delightful meal for yourself, as the kitchen is tiny but workable.