Thank you all of you for your consideration and answers

Usually remaining habitat

You can use the guidelines for application of IUCN Red List criteria at regional and national levels that are downladable for free at: https://www.iucn.org/content/guidelines-application-iucn-red-list-criteria-regional-and-national-levels-version-40

In fact, Red List criterion B, which principally uses distribution data, is the most widely used to assess conservation status.

Furthermore, there is some R packages (e.g. red, ConR,...etc) which facilitate species Red List assessments according to the IUCN categories and the accompanying criteria. Generally, these packages are able to perform large-scale multispecies calculations of the three main criterion B parameters (i.e. extent of occurrence (EOO), area of occupancy (AOO) and an estimate of the number of locations) and provide preliminary conservation assessments using an automated batch process. All you need is reliable georeferenced distribution data about species.

Follow the guidelines as stated by Idriss Bouam.

You field statistices should also be exhaustive and sincerely done

Yes. Follow the IUCN Red List Criteria.

The IUCN Red List criteria is provided. However, specific exhaustive and comprehensive monitoring is required to arrive at the decision. E.g. using population viability models. For instance:

Article Estimating population extinction thresholds with categorical...

Thank you all of you for your consideration and answers