The Scroll of Transcendence gives you the state amount of skill gain outright, so long as you have room for it under the caps for the skill and total skill.
So, an SOT of 0.4 skill in Mace fighting would raise your skill by 0.4; this skill gain will mess up any guaranteed gain you might have been able to get from time passes since last gain, so go get a normal gain in before you use one.
A Scroll of Alacrity gives you a 15 minute window of gains where you gain 0.2 to 0.5 skill for every skill gain you MIGHT get. Note that you might not actually GET a gain in that time period, unless you go out of your way to force a gain. These work best for crafting skills, where you can get multiple gains in a short period through massive use of the skill.
The 15 minute time only runs while the character is logged in, so for non-combat skills other than crafting, you can log in at intervals based on the Guaranteed Gain timer, do the skill for the GGS gain (which will be 0.2 to 0.5 skill), log back out at home or in an inn, then repeat, getting 20-30 enhanced gains (But at the expense of not playing the character over those weeks, except for the login).
This second method of using the Alacrity scrolls works best for truly horrific skills to grind, such as bard skills (over 90 real skill, you can use the bardic specializations from Britain to have gain chances without having to leave the house to find suitable targets for the standard version of skill use).