You will receive credits on a monthly basis from the council and your monthly income is dependent on how many satellites you have in orbit and how many countries are currently still part of the council. You can earn additional money throughout the month as rewards from completing certain missions, fulfilling council requests, and by selling materials and alien corpses on the grey market.
1000 a month may come naturally, however if you find you are struggling, save your game then start selling most of your equipment/recovered tech on the grey market. Once the achievement unlocks, reload the save.
TIP: Playing on Easy or Normal difficulty will make this achievement easier to unlock, as it will be easier to manage the panic levels and allow for continued full support from the council each month.
Also having a starting base in Africa will increase your monthly council funding by 30%.