How long have you been using that stack? If you've been using it for 8-12 weeks, then give yourself a 4-week break before starting on Erase/Shift. If not, and using the Erase/Shift stack would take you up to 8-12 weeks in total re usage of both stacks, then you can start straight after finishing your current one.
If you want fat loss, I recommend reading The 3 Keys to Fat Loss to more comprehensive advice and information re nutrition, supplementation, and training for effective and successful fat loss.
All the best in your endeavours!