May involve purchasing land, planting trees, digging ponds etc. Advice of an ecologist will be necessary to maximise utility to wildlife. Particular effort may be justified if the reserve can home endangered species.
The value of this project is dependant on the reserve having time to become established and continue a considerably long while afterwards. The agent(s) of this project will therefore take care to ensure protection for the reserve continues after their own death(s). Perhaps a charitable foundation will be suitable for this purpose?*
If humans are to be allowed freely into the reserve, then it may be much easier to acquire the funds in order to purchase the land: public donations may be received. Help with physical work may be available from conservation groups.
* Care must be taken if an existing charity is used for this purpose. One nature reserve, which was left to the Royal Society for the Protection of Birds on the death of its owner, was sold by them to a developer who built a car-park on it!