As of mid-December 2018, 59% of Republicans thought it was 'very likely' that Trump would get Mexico to pay for the wall (question 6A), and 9% believe he as 'already accomplished' this.

As a comparison, in April of 2017, only 46% of Republicans thought it was 'very likely' (question 4), 47% in November of 2017 (question 6), and in March of 2018, it was up to 54% (question 6A). So if anything, Republicans increasingly think he's going to be able to make Mexico pay for it.

As for specific method? Fox News is probably the best source you'll get, and they cite the memorandum Trump sent out.

As of mid-December 2018, 59% of Republicans thought it was 'very likely' that Trump would get Mexico to pay for the wall (question 6A), and 9% believe he has 'already accomplished' this.

As a comparison, in April of 2017, only 46% of Republicans thought it was 'very likely' (question 4), 47% in November of 2017 (question 6), and in March of 2018, it was up to 54% (question 6A). So if anything, Republicans increasingly think he's going to be able to make Mexico pay for it.

As for specific method? Fox News is probably the best source you'll get, and they cite the memorandum Trump sent out.