Help me please.

mightyMouse Good tip for tensioning chain when putting rear wheel back on:

Wedge a piece of cloth (usually my cloth tool bag) between seat tube and wheel by putting in on tyre and turning wheel toward BB. Once you have the wheel in correct position you can tighten wheel nuts w/o worrying about losing tension. Much less faff than Sheldon's method.

+1

I use a tea towel.

Sheldonger.