I apologize for the video playing with such high speed - Reflector had bas support for IOS 8.
First you sims have to be partners. They reach this relationship state by repeatedly doing romantic interaction towards each other. All romantic interactions are marked with purple color.
When they are partners and you are ready to propose; mark one of your sims and click on the other sim and select the option Propose marriage. A list will open up on the right where you can select ring. I always go for one of the 2 first options that you only need simoleons for but then you might end up with a turned down proposal.
If at first you don't succeed simply try and try again until the proposal is accepted.
Finally the sim will accept and your couple will now have relationship status Engaged.
Keep performing romantic interaction (why not some woo-hoo?) to max up this relationship scale and you can then get married.
When you have filled out the scale for relation Engaged the option Get married becomes available.
To get married; mark one of your
sims and click on the other sim and select the option Get married.
They will hold hands and celebrate and their relationship is now listed as married.
Sims have to be married in order to be able to have babies.