start with the first step; begin with what is at hand
Proverb meaning that when undertaking a large project, one should start with immediate, concrete actions rather than waiting for perfect conditions. Often used to encourage taking initiative.
Even big reforms should start with immediate steps, in the spirit of 'start with the first step.'
the one who proposes it should do it first
Proverb meaning that the person who suggests an idea should be the first to act on it. This sense is less common than sense 1 and may be considered a secondary interpretation.
Since they say 'he who first suggests it should be the first to do it,' you, who proposed it, should show us how it's done first.