Easiest way would be to start as early as possible. And actually, it'd be far easier than you might imagine. Simply have the Central Powers win WW1, and subsequently, with its full recognition as an ally of both the German Empire and the Ottoman Empire, have the Dervish State either get granted full sovereignty over British, French and Italian Somaliland (along with keeping a hold of the Dervish State's strongholds of Ogaden and the Haud region) in the aftermath of WW1, or by going on the offensive during WW1 and opening up a Somalian theater of WW1 while the Triple Entente colonial powers are occupied elsewhere, as part of a deal in which they'd be granted suzerainty over British/ or French Somaliland, in much the same way as Iyasu V of Ethiopia was offered French Somaliland (Djibouti) by the Germans in 1915, in return for invading the Sudan with 50,000 soldiers.
And since Iyasu V was the only Ethiopian monarch to recognize the Dervish state's independence, and even ally with it for the brief period he was in power, this would work best if both went in concert- with both the Ethiopian Empire and the Dervish State entering WW1 as agreed, on the side of the Central Powers, as mutual allies against the Entente imperialist colonials, with the Ethiopians getting Italian Somaliland and South Sudan in their half of the bargain, and the Dervish State getting French and British Somaliland, along with the Somali region which they'd already controlled for a quarter of a century anyway, in their half of the bargain.
How much manpower and materials would the Entente have been able to spare to deal with it, and would it have been anywhere near enough? With access to the Red Sea hampered by the Ottomans on one side and the Ethiopians and Dervishes on the other, how much harder would it have been for the Entente to support their continued war efforts? And even if the Arab revolt did happen in this TL, since the Dervish State was one of the most popular and famed in the Islamic world at the time, and the Yemenis were its closest diplomatic and military allies, wouldn't it only serve to strengthen the Somali Dervish State still further?
EDIT: Oh, wait, post 1945 POD. Never mind then...