I have the following advice: Write simple sentences, in an active voice, and present tense if possible.

This advice may not work under all circumstance, but it is a useful start for a non-native speaker. Simple sentences avoid issues that required more knowledge of grammar. However, a text of only simple sentences may become somewhat boring in the end. If you want to write a compound sentence, think about the principle of attention loss, so it is wise to start with the most important stuff.

Consider the following sentence (part of a methodology discussion):

Furthermore, since I am investigating the returns of portfolios comprised majority of European stocks I use the 10-year German Bund yields as the risk-free rate of return.

I recommend to rewrite this as follows, and change the order because the choice of the risk-free rate is the main message for the reader:

I use the ten-year German Bund yield as the risk-free rate because the portfolio consists largely of European stocks.

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