Modulation upsampling method
You have two methods. The second one is right, because it's actually upsampling: inserting zeros, and then suppressing images (what you call aliases here is actualyl called images) through the desired filter.
The first one is actually the same, but instead of the RRC, you use a rectangular filter: When you think about it, holding the last value is like inserting zeros, and then convolving with a 1-amplitude rectangle that is as long as the number of zeros you've inserted.