I was involved in producing a resistant tumor (myeloma) cell line previously.
What we did is to start with low concentrations of the drug, slowly (montholy) increasing it over time in small steps until getting a cell line resistant to the desired concentration of the drug. It is necessary to always add the respective drug concentration to the culture medium, otherwise the cells might lose their resistance again and change their gene expression profile.
The level of resistance can be checked by calculating the IC50 on a regular basis.
This method requires several months of time, but it worked well for us.