How do you do an estimate?

BY USING CENTRAL LINE METHOD FOR MATERIALS AND LABOUR WORK WOULD BE CALCULATED ACCORDING TO SITE CONDITION AND SPEED OF LABOURS

Estimation is based upon the quantum of work , rate analysis , deployment of skilled or unskilled labour , Cost of materials be used keeping in view the Location and access to the work location, availability of material , Labour & T&P.Evaluation of overheads , contingencies and expected profit/ Premium for the contractor. 

You must understand the Contract Doc , Specs and drawing as per request the material for the pay item and also you have to know the present market price from the different suppliers.
Any pay item you estimate before check how many items included with that item.

In my opinion i think you should have the documents and the drawings also you have to know present market rate for the item you required in this estimation.

Based on the approved drawing as stated in the plan we can calculate the estimated materials and also the flow of activities to compute the duration of the project.

- Extracting the scope of work based on reviewing and studying the tender documents.
- Coordinating with the QS in terms of issues related to Quantities Calculations and cross checking.
- Corresponding with subcontractors / suppliers to obtain prices & quotations for completing the tender pricing .
- Estimates Direct cost (man-hour, labor, equipment, material cost) - In general ,Study and Analysis of the tenders , preparation Bill of quantities, study the technical specifications and drawings , preparation of an approximation budget included materials and labor costs and all expenses related to tender for all trades , after that We analyze the offers and choose the best and competitive offer, to be later issued purchasing order of materials that have been selected.

also you call quantity surveying , it aims to determine the cost of the project and simply you can define it as mathmatics operation that uses the sizes , dimensions and spacings shown in the construction plans in order to determine the quantity of materials ( cement , reinforcement , ....
etc) used in the construction and following the process to produce the final quantity and work table , to judge that the estimation process succeded , the estimated cost should not exceed the ratio10% more or less of the actual cost ..