Start a new game, choose a campaign that shows as "difficulty: easy" on the campaign selections screen.
On the bottom of the screen you will see "game rules" click on this and enable Ironman. Make sure difficulty is set to normal or higher in order to obtain achievements and then click on close. Start the campaign.
If your character is married:
- Click on character picture in the bottom left of your screen
- right click on spouse and choose divorce
Or else skip to this part:
- Right click on your character and choose, find spouse
- In drop down menu choose 'Opinion of You'
- Select the first spouse from the list after its sorted
- When you pick a spouse be sure to click on large text box on the right and not the portrait picture of spouse on the left
- Choose send proposal
After the proposal is sent you can increase game speed by pressing '5' and it shouldn't take more than 10 seconds to see that the marriage proposal was accepted. I used Ivar the Boneless campaign for this.