You should be able to conquer Portugal by mid or late 1938. Later convert 6 of them to the marine template. Convert all your infantry brigades to this template.
Now for the army start training the cheapest infantry(consisting of 3 infantry) divisions in batches of 3.
Do not build any submarines till you research level 3 submarines. It should finish by the time you declare war on Portugal. Head to Liberate Portuguese Colonies in the focus tree. Prepare defensive units on the Portuguese border. Prepare naval invasions to the Portuguese territory in Africa leading to the Portuguese mainland in Europe. Follow the focus tree to become communist and get the Anti Colonialist Focus once free from the United Kingdom. Build a large submarine fleet and naval dockyards.
Immediately research marines, submarines, and naval invasions. A faction member has to control the Greater London Area as well. This achievement requires South Africa complete the focus 'Anti-Colonialist Crusade', become leader of a faction and release Angola, Zaire, Kenya, Mozambique and Zimbabwe.
