It's a good idea to write a quick plan before you start writing your letter. Take this question for example:
Here's my quick plan:
- Informal style - friend's name can be John.
- Quick greeting and reason for writing.
- Neighbour at number 10 has keys. Visit any time after 5pm.
- Instructions about turning the heating off and opening the windows.
- Good local restaurant and a traditional festival in the town during his visit.