Just like in any other bleeding patient, get access and give fluids/blood products as needed.  Then you need treat the underlying cause!

4 Sources, 4 Ts: Tone, Tissue, Trauma, Thrombin (coagulopathies)

Uterine atony is by far the most common cause of bleeding so we’ll concentrate on that:

• Do aggressive uterine massage and even bimanual internal massage
• Give oxytocin. Remember this dosage: 40 units in a 1 L bag wide open or just give 10 units IM

The combination of these two actions should buy you time until OB arrives.  If bleeding continues, move on to secondary therapies:

• Use carboprost tromethamine (Hemabate) 250mcg IM (avoid in asthmatics). Can repeat every 15 - 90 minutes with a max of 2 mg.
• Methylergonovine (methergine) 0.2 mg IM (watch for HTN)
• You can place a Bakri balloon within the uterus and fill with 500cc of saline to induce tamponade
• The WHO suggests using TXA, 1g IV, if none of the above work

References:
Belfort, MA. Management of postpartum hemorrhage at vaginal delivery. In: UpToDate, Post TW (Ed), UpToDate, Waltham, MA. (Accessed on Feburary 2, 2017.)
Postpartum Hemorrhage - WikEM. N.p., n.d. Web. 02 Feb. 2017.