I am not that familiar with the mechanics of Star Frontiers, but I have a process that I think is relatively universal, at least for translating into d20.
1) Make a photocopy of the module and read through it with a highlighter in my hand. I mark anything that involves game mechanics.
2) Decide what level party you want playing the module.
3) Go through each highlighted bit and decide what the nearest equivalent is. This mostly involves assigning DCs to skill checks. Read through the skill description in your d20M core book and/or the Difficulty Class Examples on p. 44. Note: don't try to match the probability of the old mechanic, just the closest sense you have of how hard it should be. When in doubt, it's a DC 15.
4) Don't convert NPCs point-by-point. Read up on them to get the broad idea, the ur-character, if you will, and then rebuild the character from the ground up in the new ruleset. Character level should be assigned based on the level of the party.
5) As much as possible, convert equipment by just finding something in d20 Modern, Future or Future Tech that is pretty much the same concept and changing the name. If you want to make adjustments, put the data you aready have in the format given in the original. I like to use Excel to create the tables, but you can also just cut-and-paste the tables out of the SRD and empty out the cells to give you the paradigm.
6) Maps may need to be re-gridded for 2-meter or 5-foot squares. It may be that you can do this by eyeball at game time, but it may also be convenient to create maps that can be printed out so that the miniatures can be placed directly on them.
7) Combats and the vital skill checks must be assigned ELs, according to the rules in d20M 204-209.
8) Wealth bonuses should be assigned by the table on 204, or by treating all ELs as CRs, calculating a new total encounter level, and then consulting the table on 204. Or by assigning whatever number first comes into your head as you drive a spike into it with a rubber mallet -- whichever method is less painful and disorienting.
More generally, I'd advise:
Don't cross-breed pigs and elephants. Make the conversion as native to the new system as possible. It's less work for you, less to explain at the gaming table, and less of a hassle for the players who have already learned the rules as written. Go native with your conversion at every step possible.